Fixed a couple inconsistencies in the MessageError<M> vtable#61

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Michael-F-Bryan wants to merge 2 commits intodtolnay:masterfrom Michael-F-Bryan:inconsistent-messageerror-vtable

+3-1

Commits on Jan 21, 2020

Mentioned why it's okay to downcast with M instead of MessageError<M>
Michael-F-Bryan
committed
Use MessageError<M> with drop_rest because it's equivalent and more consistent
Michael-F-Bryan
committed