Add optional Validate() error method

[peczenyj]

port of https://github.com/dmarkham/enumer/pull/82

this pull request will adds an optional method Validate() error -- and this is a very common interface -- that return an error if the IsA<Type>() bool returns false.

The advantage over IsA<Type>() bool is the interface. Makes easier to integrate with other tools such as validators

[peczenyj]

conflict solved