Change error message for Play JSON to be like Play

[eddsteel]

In Play JSON, a small number of constant strings are used for validation errors. These can then be localized. It also enables non-Play code to use this message, combined with the path specifying the bad field's location, to generate useful messages.

This change modifies the Play JSON Reads for enums to use strings already in use in Play. Although on their own they are less informative than what's there now, they fit in better with Play and when combined with paths can be used to make more informative messages. (This is our use case. We're not using the play framework but expecting Play JSON behaviour. With current enumeratum behavior we have to prefix-match the error string to generate our errors, but match constant strings for other fields).

If you prefer I can make this an optional thing, like case-insensitivity, but I thought I'd propose this simpler change first.

[coveralls]

Coverage remained the same at 100.0% when pulling 9a2c661 on eddsteel:eddsteel/play-validation-message into 6d7e6cf on lloydmeta:master.

[lloydmeta]

Hey there, I think think this change is beyond reasonable. I'm not too worried about breaking backwards compatibility because:

1. This is about changing the error case, so success cases will not change.
2. As you mentioned, it fits how Play-JSON handles errors better.

Thanks !