Better error handling

[squito]

if something goes wrong during parsing, the error messages are often unclear.

The good news is, at the very least you always get a usage message. But I think lots of error messages get swallowed, some aren't the most meaningful, etc.

Also, its not clear what kind of error a user should throw in their own parsers & validation functions to get nice messages.

maybe this can be resolved just be documenting existing functionality ...