ENH: Better error catching when loading plugins

[TEParsons]

Currently, because we use return False rather than continue when an entry point fails to load, if one entry point fails then none of the rest of the entry points for that plugin are registered. There also isn't any detail given, so the logs just look like:

ERROR Failed to load <entry point> from <plugin-name> for unknown reason. Skipping

This PR means that when an entry point fails, only that specific entry point is skipped, and we'll log the reason.