feature: improved exception message when there are multiple resource IDs with the same name

[Qonstrukt]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
This clarifies the exception you get when ControlFetcherMixin collects the names of resource ID's in your layout files. It shows you for which ID's it's exactly failing so it's easier to find which names are the culprit.

What is the current behavior? (You can also link to an open issue here)
When you have duplicate names with different casings in use as resource ID's, you get a fairly generic ArgumentException which is hard to understand without diving into the code of ControlFetcherMixin.

What is the new behavior (if this is a feature change)?
We throw a more explanatory InvalidOperationException which shows which names should be fixed to get things going.

Does this PR introduce a breaking change?
Nope, unless someone is already catching the ArgumentException themselves, but I can't imagine anyone relying on such functionality.

Please check if the PR fulfills these requirements

• The commit follows our guidelines: https://github.com/reactiveui/reactiveui#contribute
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:
As reported in #1275, and we ran into this ourselves as well. Cost multiple of our engineers a good couple of minutes to figure out what's wrong.

[coveralls]

Coverage remained the same at 66.782% when pulling efc930a on Qonstrukt:duplicate-key-exception into f766ddd on reactiveui:develop.

[coveralls]

Coverage remained the same at 66.824% when pulling 54b16c0 on Qonstrukt:duplicate-key-exception into 2379efc on reactiveui:develop.

[ghuntley]

Thanks @Qonstrukt!