debug: New pylint => new warnings => new cleanups

- Replace general "Exception" with "GdbServerError" in gdbserver.py for when no
samples are collected
- Replace general "Exception" with "TargetsException" in targets.py for XLEN
mismatch
- Introduce "TestLibError" exception in testlib.py and replace general
exceptions in various locations
- Update pylint.rc to remove overgeneral-exceptions warning