add checking and error handling for encrypted keys and pass_phrases

this fixes https://github.com/opscode/cheffish/issues/9

This is my first attempt at fixing this and want to start a discussion to make sure i'm going in the right direction. Any feedback is appreciated!
Is there a more specific way to test for this?  The key_contents could contain the phrase ENCRYPTED in the key itself.  Maybe make sure it's on the first line?