Resolve change in InvalidNumber handling in writers

[jsiirola]

Fixes #3386 .

Summary/Motivation:

This resolves #3386 by making it so that both InvalidNumber.__str__ and InvalidNumber.__repr__ raise exceptions. It was relatively straightforward to remove the use of string conversions in the tests so that we no longer needed to cast InvalidNumber to a string.

Note that this includes an exception to the "str(InvalidNumber) raises an exception rule: if we are in the middle of processing an exception, this does not raise another exception and instead returns the old string representation.

Changes proposed in this PR:

• Update InvalidNumber so that both str() and repr() raise exceptions
• Clean up how some deprecation warnings generate warnings about NaN
• Update tests

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[blnicho]

I like this solution!