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