Enabling pattern matching on JWTErrors

[wcrestfield]

As is JWTErrors are can not be pattern matched with generic errors. This keeps us from catching specific JWTErrors.
Currently we must:

do {
	let jwt = try JWT<MyClaims>(jwtString: token,  verifier: verifier)
} catch let error as JWTError {
	switch error {
	case .invalidJWTString:
		handleInvalidJWT()
	case .failedVerification:
		handleFailedVerification()
	}
}

With this change, it allows the following:

do {
	let jwt = try JWT<MyClaims>(jwtString: token,  verifier: verifier)
} catch JWTError.invalidJWTString {
	handleInvalidJWT()
} catch JWTError.failedVerification {
	handleFailedVerification()
}

[CLAassistant]

All committers have signed the CLA.

[ianpartridge]

Thanks for your PR. This looks like a good idea to me, we'll get it reviewed ASAP. It would be nice to add a test or two as well.

[Andrew-Lees11]

This looks good to me as well. Just a quick note that in your new code example it would have to be:

do {
	let jwt = try JWT<MyClaims>(jwtString: token,  verifier: verifier)
} catch JWTError.invalidJWTString {
	handleInvalidJWT()
} catch JWTError.failedVerification {
	handleFailedVerification()
} catch {
       handleOtherError()
}

So that the catch is exhaustive.