Create a custom exception for invalid driver error | close #348

[RaulShinaede]

Thank you for your contribution!
Before submitting this PR, please consider:

• If you are fixing something other than a typo, please always create an issue first. Otherwise, your PR will probably be rejected.
• You have added unit tests which (a) pass and (b) sufficiently cover your changes

[jamesmh]

Thanks! Could you add at least a couple of tests against the AddMailer method?

1. Passing test against one of the valid drivers
2. Test again a missing driver and validation the proper exception is thrown

👌