Add errors module API docs

dependency_injector/errors.py

@@ -2,8 +2,21 @@
 
 
 class Error(Exception):
-    """Base error."""
+    """Base error.
+
+    All dependency injector errors extend this error class.
+    """
 
 
 class UndefinedProviderError(Error, AttributeError):
-    """Undefined provider error."""
+    """Undefined provider error.
+
+    This error is used when provider could not be defined, for example:
+
+        - provider with certain name could not be defined
+        - catalog's name of the certain provider could not be defined
+        - etc...
+
+    Also this error extends standard :py:class:`AttributeError`. This gives
+    possibility to use it correctly with ``__getattr__()``.
+    """

docs/api/errors.rst

@@ -3,3 +3,4 @@
 
 .. automodule:: dependency_injector.errors
     :members:
+    :member-order: bysource