add helpful exceptions for userland / traits

[jrushlow]

• AlreadyVerifiedException::class - Used if a users has already been verified.
• InvalidSignatureException::class - Used anytime Helper::isValidSignature returns false.

These exception as more appropriate for user land code. But may prove useful in the yet to be written, VerifyEmailControllerTrait::class (PR #1)

Use case:

 if ($user->isVerified()) {
    throw new AlreadyVerifiedException();
}

if (!$helper->isValidSignature($request->getRequestUri(), $user->getId(), $user->getEmail())) {
    throw new InvalidSignatureException();
}

[weaverryan]

If these were used in the trait in #1, then I'm all for them. But I just added some comments there that may make them not needed :)

[jrushlow]

#1 and this PR are sort of in limbo. I'm coding these out in maker / test app. Converted this one to a draft until I have a better idea of what the implementation looks like.

Will provide the full context in the next go around...

[jrushlow]

no longer relevant