propagate errors (#15)

This PR fixes errror propagation. At the moment commands like `uenv run store.squashfs -- my_cmd` will always look successful, independent of the status of `my_cmd`.

With this PR it will be possible to propaget the error, and chain commands in the shell with `&&` or `||`.

There is a tight integration of `uenv-impl` and the `uenv` function, because `uenv-impl` sets variables that will be returned from the `uenv` function.
This is not great, but I could not figure out a way how else I could make it happen.

Here are two examples that would work with the suggested changes:
```bash
if uenv run store.squashfs -- false ; then
  echo "Success status"
else
  echo "Failure status"
```

and also
```bash
if uenv start store.squashfs ; then
  echo "Success status"
else
  echo "Failure status"
fi
```

The latter will have the exit status of the last run command, which is the default behaviour for subshells, where the last command's exit status defines the subshell's exit status.

activate

@@ -6,6 +6,8 @@ export UENV_CMD=@@impl@@
 export UENV_VERSION=@@version@@
 
 function uenv {
+    local _last_exitcode=$?
+
     function uenv_usage {
         echo "uenv - for using user environments [version @@version@@]"
         echo ""
@@ -31,6 +33,7 @@ function uenv {
     fi
 
     unset -f uenv_usage
+    return $_exitcode
 }
 
 export -f uenv

uenv-impl

@@ -15,6 +15,7 @@ import subprocess
 VERSION="@@version@@"
 
 shell_noop=" :"
+shell_error="local _exitcode=1"
 
 def make_argparser():
     parser = argparse.ArgumentParser()
@@ -384,7 +385,7 @@ def generate_command(args):
     env = environment()
     if env.old_api:
         print_error(f"the version of squashfs-mount on this system is too old.")
-        return shell_noop
+        return shell_error
 
     if args.command == "run":
         return generate_run_command(args, env)
@@ -400,7 +401,7 @@ def generate_command(args):
         return generate_modules_command(args, env)
 
     print_error(f"unknown command '{args.command}'")
-    return shell_noop
+    return shell_error
 
 """
 uenv start gromacs.squashfs ddt.squashfs
@@ -538,7 +539,7 @@ def generate_run_command(args, env):
     print_debug(f"parsing run command with arguments: {args.command}")
     if env.active:
         print_error("a uenv is already running")
-        return shell_noop
+        return shell_error
 
     images, cmd, cmdargs = split_runline(args.runline)
 
@@ -548,7 +549,7 @@ def generate_run_command(args, env):
 
     mount_pairs=parse_image_descriptions(images)
     if mount_pairs==[]:
-        return shell_noop
+        return shell_error
 
     mount_string = ' '.join(mount_pairs)
     cmdargs_string = ' '.join([f'"{arg}"' for arg in cmdargs])
@@ -558,18 +559,19 @@ def generate_run_command(args, env):
     return [f"export UENV_MOUNT_FILE={main_mount[0]}",
             f"export UENV_MOUNT_POINT={main_mount[1]}",
             f"squashfs-mount {mount_string} -- {cmd} {cmdargs_string}",
+            'local _exitcode=$?',
             "unset UENV_MOUNT_FILE; unset UENV_MOUNT_POINT;"]
 
 
 def generate_start_command(args, env):
     print_debug(f"parsing start command with arguments: {args.image}")
     if env.active:
         print_error("a uenv is already running")
-        return shell_noop
+        return shell_error
 
     mount_pairs=parse_image_descriptions(args.image)
     if mount_pairs==[]:
-        return shell_noop
+        return shell_error
 
     mount_string = ' '.join(mount_pairs)
 
@@ -578,6 +580,7 @@ def generate_start_command(args, env):
     return [f"export UENV_MOUNT_FILE={main_mount[0]}",
             f"export UENV_MOUNT_POINT={main_mount[1]}",
             f"squashfs-mount {mount_string} -- bash",
+            "local _exitcode=$?",
             "unset UENV_MOUNT_FILE; unset UENV_MOUNT_POINT;"]
 
 def generate_modules_command(args, env):
@@ -586,7 +589,7 @@ def generate_modules_command(args, env):
 
     if not env.active:
         print_error(f'there is no environment loaded')
-        return shell_noop
+        return shell_error
 
     # generate a list of all the mounted environments that provide modules
     module_envs = [
@@ -631,7 +634,7 @@ def generate_modules_command(args, env):
                             and (e.is_native_mounted)]
                 if len(matches)==0:
                     print_error(f"no uenv matching {i} provides modules")
-                    return shell_noop
+                    return shell_error
                 print_debug(f"  uenv {i} mounted at {matches[0]}")
                 mounts.append(matches[0])
 
@@ -649,11 +652,11 @@ def generate_view_command(args, env):
 
     if not env.active:
         print_error(f'there is no environment loaded')
-        return shell_noop
+        return shell_error
 
     if env.loaded_view is not None:
         print_error(f'a view is already loaded: {env.loaded_view}')
-        return shell_noop
+        return shell_error
 
     uenv = env.uenvs[0]
     name = args.view_name
@@ -668,6 +671,7 @@ def generate_view_command(args, env):
                 f"export UENV_VIEW={uenv.mount}:{name}",]
     else:
         print_error(f'the view "{name}" is not one of the available views: {available_views}')
+        return shell_error
 
     return shell_noop
 
@@ -734,9 +738,9 @@ def generate_stop_command(args, env):
 
     if not env.active:
         print_error(f"there is no running environment to stop")
-        return shell_noop
+        return shell_error
 
-    return "exit 0"
+    return "exit $_last_exitcode"
 
 
 if __name__ == "__main__":