Show original error message when plugin fails to load

[rwedge]

Problem Description

Currently when an entry point plugin fails to load, a UserWarning is diplayed with the following message template:

'Failed to load "{entry_point.name}" from "{entry_point.module_name}".'

This message template does not help much in ascertaining why the entry point failed to load

Expected behavior

The message template should be update to also include the error message that caused the failure, after the existing text.

'Failed to load "{entry_point.name}" from "{entry_point.module_name}" with error: {error_msg}'