Add an ImproperlyConfigured exception.

- To be used to signal when a type is being used without a
   required library.

sqlalchemy_utils/__init__.py

@@ -1,3 +1,4 @@
+from .exceptions import ImproperlyConfigured
 from .functions import (
     sort_query, defer_except, escape_like, primary_keys, table_name
 )
@@ -26,6 +27,7 @@ __version__ = '0.14.4'
 
 
 __all__ = (
+    ImproperlyConfigured,
     coercion_listener,
     sort_query,
     defer_except,

sqlalchemy_utils/exceptions.py

@@ -0,0 +1,8 @@
+"""Global SQLAlchemy-Utils exception classes.
+"""
+
+class ImproperlyConfigured(Exception):
+    """
+    SQLAlchemy-Utils is improperly configured; normally due to usage of
+    a utility that depends on a missing library.
+    """