Throw instance of exceptions.NoUniqueMatch and not the class

I think there is an error in line 195 in base.py: It seems that the class exceptions.NoUniqueMatch is thrown and not an instance of that class. This leads to an incomplete error message like "ERROR: <attribute 'message' of 'exceptions.BaseException' objects>".

We throw now an instance of exceptions.NoUniqueMatch with a explaining message.