[iso-core] Fix imap error detection/coercion (again)
[iso-core] Fix imap error detection/coercion (again)

Summary:
If for whatever reason we passed an error with a `source` property
we weren't accounting for in `convertIMAPError`, we would return an
`undefined` error. Bad!

Instead, just return the original error

Test Plan: manual

Reviewers: spang, halla, evan, mark

Reviewed By: evan, mark

Differential Revision: https://phab.nylas.com/D4037

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