Raise Exception when MailerPreview Method Fails to Return a Mailer

[jonmchan]

Small developer quality of life feature suggestion:

When the MailerPreview method does not return a proper Mailer object, we get a non-obvious error as the gem tries to use the returned object as a mailer:

NoMethodError (undefined method `multipart?' for nil:NilClass):

Might be helpful to raise an error earlier when the method does not return the proper object such that developers don't have to dig through what is calling multipart?.