Context parsing should do its own error handling

[dburgener]

Currently Context::try_from() (both the String and &str variants) return () on error and trust the level above to write a good error message. This discards information about what failed in parsing. try_from() should return something more helpful, and the level above can decide what it wants to do. Perhaps rather than a full error, try_from() might return just a help string. Or maybe just a normal CompileError works. More analysis should be done on callers.