Fix error in grease pencil flip color operator

- Used try/except instead of a poll function.
- The error case referenced a non-existent error handling module.

Prefer poll functions over exception handling where possible,
also having an operators logic in a try block isn't good practice
as it can hide more serious errors in the code.

Note that duplicate pencil settings access should be moved into a
utility function. This can be part of a separate cleanup.

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