Add try/except around all plugin loads

[tbttfox]

Checklist

• I have read the CONTRIBUTING.md document
• I formatted my changes with black
• I linted my changes with flake8
• I have added documentation regarding my changes where necessary
• Any pre-existing tests continue to pass
• Additional tests were made covering my changes

Types of Changes

• Bugfix (change that fixes an issue)
• New Feature (change that adds functionality)
• Documentation Update (if none of the other choices apply)

Proposed Changes

We can't trust plugins to load, so wrap all entry point loads in a try/except block.
Note: If we try to run preditor for the first time in a python instance that doesn't have QScintilla installed, it will crash because it tries to load the scintilla workbox.

[MHendricks]

Would you want to see the traceback as part of this logging message instead of just logging the plugin name?

_logger.warning('Unable to load editor {}'.format(ep.name), exc_info=True)

import logging
try:
    raise Exception("The plugin failed for reasons")
except Exception:
    logging.warning("unable to init plugin", exc_info=True)

output:

WARNING:root:unable to init plugin
Traceback (most recent call last):
  File "<WorkboxSelection>:misc/Workbox46", line 17, in <module>
Exception: The plugin failed for reasons