Throw AmbiguousObjectException during resolve if its ambiguous

Throw AmbiguousObjectException during resolve if its ambiguous

Its wrong to return null if we are resolving an abbreviation and we
have proven it matches more than one object.  We know how to resolve
it if we had more nybbles, as there are two or more objects with the
same prefix.  Declare that to the caller quite clearly by giving them
an AmbiguousObjectException.

Change-Id: I01bb48e587e6d001b93da8575c2c81af3eda5a32
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>