Sanity check for user input

[ferraith]

The user input passed on the command line or in config file should be validated by setuptools-pyecore. A sanity check for the following parameters should be added:

• user-modules
• output

If the path to a output directory or to a user module isn't valid (e.g. doesn't exist) a warning should be thrown. For an user-module the code should still be generated without passing the user-module path to the EcoreGenerator. For an invalid output path the default output folder should be used.