Fix #536 - add help for format string parsing error

[jonathanpallant]

1. Replaces Cow<str> with an enum Error to accurately describe the parse error
2. Uses thiserror to convert the Error enum into strings, which match the strings previously returned from the parse function
3. Uses that enum to add help text for Error::UnknownDisplayHint.

Closes #536

[Urhengulas]

bors r+

[bors]

Build succeeded:

• ci