v0.5.1: Cleaner messages and sane map mutation

This release addresses a quality-of-life issue with error messages and a bug with fctx maps being mutated unexpectedly.

• Fault's top-level .Error() method will now filter out non-messages like <fctx> so instead of some error: <fctx>: <fctx>: <fctx>: <ftag>: root cause you'll just see some error: root cause
• The underlying map used to store the key-value metadata in fctx context and error objects is now copied when wrapped so the original isn't modified.