Correct ObjectWalk error message when bad object is found

Instead of including "ObjectId[SHA-1]" in the message, just
us the formatted SHA-1 name of the object by calling name().

Change-Id: I0d1d0e8207f8a3f02188e60242e4e9bf7420e88f
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>