add helpful error to ziti-edge-tunnel enrollment

[dovholuknf]

when a user executes ziti-edge-tunnel with a single parameter to enroll nothing seems to happen.

steps to reproduce:
issue: ./ziti-edge-tunnel enroll <some jwt>

observation:
nothing happens. no error, no output. also "./ziti-edge-tunnel enroll -j " does not tell me i need "-i" for output

expectation:
a helpful error indicating i have not supplied the proper arguments to enroll

[qrkourier]

I reproduced this problem today with 0.17.16 with this command. There was no output at all, but the error exit code was 255.

$ ./ziti-edge-tunnel_0.17.16 enroll -k bootstrap.key -c bootstrap.crt -j registration.jwt -n "acme-device-1234"