👌 IMPROVE: MultipleEntryPointError message

[chrisjsewell]

Include the actual entry-points object in the exception message,
to allow debugging of what packages are causing the clash.

fixes #696

for example, an error message would be:

MultipleEntryPointError: Multiple entry points 'name' found in group 'aiida.groups': (EntryPoint(name='name', value='aiida.orm.groups:Group', group='aiida.groups'), EntryPoint(name='name', value='other.groups:Group', group='aiida.groups'))

[sphuber]

Uh-oh, trouble in paradise? Did you actually encounter this locally? Any sign of a problem with the switch from reentry to importlib_metadata?

[chrisjsewell]

h-oh, trouble in paradise? Did you actually encounter this locally?

Oh no, nothing like that; was just searching the issues for any related to reentry which could now be closed

(see also #3919, #4455)