Add ConfigurationError exception class

mkdocs/config.py

@@ -2,6 +2,8 @@
 
 from mkdocs import utils
 from mkdocs.compat import urlparse
+from mkdocs.exceptions import ConfigurationError
+
 import errno
 import os
 import sys

mkdocs/exceptions.py

@@ -0,0 +1,3 @@
+class ConfigurationError(Exception):
+    """Error in configuration"""
+    pass