log warning if MySQLdb python module is missing when the mysql_database ...

...state module is used

I was learning how to install and setup MySQL with Salt, and at one point couldn't figure out why I couldn't get Salt to create a database in a very simple test.

After spending a bunch of time trying to debug things, I finally caught this important note right at the top of http://docs.saltstack.com/ref/states/all/salt.states.mysql_database.html::

    This module requires the MySQLdb python module

I think some sort of log message would have helped me figure out what I was missing pretty quickly, and that's what this pull request tries to do...!