Report better errors when a project revision is invalid

If a manifest specifies an invalid revision property, give the
user a better error message detaling the problem, instead of an
ugly Python traceback with a strange Git error message.

Bug: REPO-2
Signed-off-by: Shawn O. Pearce <sop@google.com>

error.py

@@ -17,6 +17,10 @@ class ManifestParseError(Exception):
   """Failed to parse the manifest file.
   """
 
+class ManifestInvalidRevisionError(Exception):
+  """The revision value in a project is incorrect.
+  """
+
 class EditorError(Exception):
   """Unspecified error from the user's text editor.
   """