Change Contract For Erlex Errors

[asummers]

Because of the history of Erlex being inside Dialyxir, we throw on errors because it was used as a sort of control flow. This is gross and nonidiomatic for cross project errors, and we should raise or provide error tuples. Because this is a breaking change to Erlex, Dialyxir will need to tolerate both style error messages. This begs a few back compat questions about if we even care to maintain and sequencing questions, as qell as poses design questions of how we actually want Erlex errors to manifest in Dialyxir for maximum flexibility.

[jeremyjh]

I'm fine with a change in contract and supporting both in Dialyxir.