Better uncoercible error messages/failure time

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When writing functions that coerce from one type to another, especially in cross-platform bindings, it is common to have a type mismatch in coercions. Currently, this error manifests as a run-time exception: Ctypes_coerce.Uncoercible.

This exception does not have enough information to be useful. Additionally, it should be possible to move many coercions to compile-time with stub gen so static run-time coercions don't potentially result in run-time exceptions.