Allow implicit conversions for NN errors -- runtime

This change allows GeneralErrors to be created from a string and allows
ExecutionErrors to be created from a string or a GeneralError.
This makes error handling more terse, removing the need for helper
functions such as makeGeneralFailure or makeExecutionFailure.

Bug: N/A
Test: mma
Change-Id: I8ed01dc6b70e493857fbe4c8fef62cba43e4ee92
8 files changed