permian doesn't fail when directed to non-existing test library dir about permian

[pholica]

After short investigation, I found the root cause to be in tclib in the way it uses the os.walk to fetch test plans, test cases and requirements: https://github.com/rhinstaller/tclib/blob/main/tclib/library.py#L13
The os.walk silently ignores exceptions according to the documentation: https://docs.python.org/3/library/os.html#os.walk
There's on_error argument which can be used to manually re-raise the exception (using single-purpose function just calling raise).
The next thing that would needed to be done is to catch the OSError exception in the Permian itself providing user-friendly error message, but that is probably just a cosmetic thing in this context.

What I'm concerned about are other situations where the on_error may be invoked - maybe broken symlinks? I'm not sure, but I'm little bit worried this may cause some headaches :)