Shorten redundant valid_uuid? method to valid?

The class name UUID defines that the subject are UUIDs. Therefore
UUID::valid? is more to the point than UUID::valid_uuid?
Converter::decode should return UUID string

To make writing other converter easier, except that Converter::decode
return an UUID string. Therefore moved decode_short_string from UUID
to Converter.
Raise InvalidUUID insteadof ArgumentError (better error handling)

To support catching only errors caused by invalid UUID use own (dedicated)
exception instead of ArgumentError.