Fixed uncaught exception when XRefMap is invalid

[PathogenDavid]

The deserialization exceptions were making it to the generic unhandled exception dialog due to the relevant catch clause looking for SystemException.

SystemException (and its complement ApplicationException) is a bit of a failed experiment from .NET Framework 1. It's no longer used for new System.* exceptions, and even before .NET Core it was applied very inconsistently. IMO the line between exceptions in the System namespace and exceptions outside of it is basically meaningless and very fuzzy regardless.

As such this PR increases the scope to just plain Exception.

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This still would have resulted in very vague low-level errors reaching the end user, so I introduced DocumentationException to denote specific errors when parsing the XRefMap.

When one XRefMap fails due to a 404 and another fails due to a DocumentationException, the DocumentationException takes priority.

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Here is the error that occurs when the documentation server responds with something other than an XRefMap:

And here's the error that occurs when the documentation server responds with something that claims to be an XRefMap but isn't unusable:

Fixes #1507

[glopesdev]

This looks great, thanks! Just very minor style tweaks for consistency with current code, looking forward to drafting together a style guide for modern .NET.

[PathogenDavid]

Fixed!

looking forward to drafting together a style guide for modern .NET.

I'll definitely have strong opinions about your try { SingleLine(); } habit. It breaks auto-formatting 😅

I can cope for now though.