Use the XML exception tag in doc comments (C#)

[kevinhartman]

Adds proper XML tags for any exceptions previously mentioned within the summary tag.

Currently, this is only done for C# projects, since most of the F# code uses implicit summary tags.

Ref #699

[bamarsha]

Thanks for doing this! It's much easier to see which exceptions a function throws now. :)

[kevinhartman]

A few TODOs prior to merge:

• Remove "Thrown if" prefix from exception tag descriptions. Otherwise, generated documentation might be redundant (e.g. DocFX will put these descriptions inside column entitled "Conditions").
• Add paramref tags for all parameter references in exception descriptions.
• Combine exception tags with the same cref (approach TBD). (see discussion)
• Add see tags for code references to exception tag descriptions.

[kevinhartman]

This one is ready for another look @SamarSha

Thanks for all of the great discussion so far!

[bamarsha]

Looks great! Thanks again for working on this!

[bamarsha]

/azp run

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