Raise a NoManifestException when the manifest DNE

When a command (eg, `repo forall`) expects the manifest project to
exist, but there is no manifest, an IOException gets raised.  This
change defines a new Exception type to be raised in these cases and
raises it when project.py fails to read the manifest.

Change-Id: Iac576c293a37f7d8f60cd4f6aa95b2c97f9e7957

error.py

@@ -21,6 +21,10 @@ class ManifestInvalidRevisionError(Exception):
   """The revision value in a project is incorrect.
   """
 
+class NoManifestException(Exception):
+  """The required manifest does not exist.
+  """
+
 class EditorError(Exception):
   """Unspecified error from the user's text editor.
   """