Add base exception class for this library

To make it easier for users of this library to
differentiate general exceptions from automaton
exceptions it is quite common to have a root exception
class that libraries use (and derive there exceptions
from); this adds that.

Change-Id: I140cb188f58849ac5caea97ed5a422375b549f6f

automaton/exceptions.py

@@ -15,23 +15,27 @@
 #    under the License.
 
 
-class InvalidState(Exception):
+class AutomatonException(Exception):
+    """Base class for *most* exceptions emitted from this library."""
+
+
+class InvalidState(AutomatonException):
     """Raised when a invalid state transition is attempted while executing."""
 
 
-class NotInitialized(Exception):
+class NotInitialized(AutomatonException):
     """Error raised when an action is attempted on a not inited machine."""
 
 
-class NotFound(Exception):
+class NotFound(AutomatonException):
     """Raised when some entry in some object doesn't exist."""
 
 
-class Duplicate(Exception):
+class Duplicate(AutomatonException):
     """Raised when a duplicate entry is found."""
 
 
-class FrozenMachine(Exception):
+class FrozenMachine(AutomatonException):
     """Exception raised when a frozen machine is modified."""
 
     def __init__(self):

doc/source/api.rst

@@ -28,3 +28,10 @@ Exceptions
 
 .. automodule:: automaton.exceptions
     :members:
+
+Hierarchy
+---------
+
+.. inheritance-diagram::
+    automaton.exceptions
+    :parts: 1

doc/source/conf.py

@@ -23,7 +23,8 @@ sys.path.insert(0, os.path.abspath('../..'))
 extensions = [
     'sphinx.ext.autodoc',
     'sphinx.ext.doctest',
-    'oslosphinx'
+    'sphinx.ext.inheritance_diagram',
+    'oslosphinx',
 ]
 
 # autodoc generation is a bit aggressive and a nuisance when doing heavy