Add neasted calling of withMadratLogging with different parameters and add warningsAsErrors parameter

[tobiasdiez]

This PR changes how withMadratLogging() behaves when calls are nested.

Previously, the first, outermost call installed the handlers. Every nested withMadratLogging() call then returned its expression immediately. As a result, its logOnly argument had no effect: all warnings and errors were handled according to the outermost call’s settings.

With this PR, each withMadratLogging() call temporarily introduces its own policy with the following update mechanism:

• An explicitly supplied nested argument overrides settings locally.
• An omitted nested argument inherits the enclosing policy.

Also this PR introduces a warningsAsErrors argument to withMadratLogging so that warnings are logged as errors with the correct verbosity handling.

[tobiasdiez]

Running buildLibrary hangs for me with:

....
─  checking tests ... SKIPPED
✔  checking for unstated dependencies in vignettes ...
✔  checking package vignettes ...
✔  checking re-building of vignette outputs (3.9s)
✔  checking for non-standard things in the check directory
✔  checking for detritus in the temp directory ...
   

any idea what's the problem?