Trigger an error when expected spec class is not found in file

fixes #354

First phpspec PR, please be gentle!

I have often come across this issue when renaming/moving a spec file and forgetting to update the classname/namespace inside.

I'm not sure throwing an exception is the correct thing to do here, if there is a more appropriate way to handle this a nudge in the right direction would be muchly appreciated.
oops, sorry.