Bad module in plugins dir makes dunes unusable

[ScottBailey]

Given a module such as: src/pugins/gotcha/gotcha.py

gotcha

dunes will fail with an exception.

Fix by:

• [x] adding exception trapping around the load_module() code in src/dunes/__main__.py.
• [x] adding a test case inside tests/test_plugin.py to verify the fix.