Fix walk_tree when facing a Lambda
Fix walk_tree when facing a Lambda

1. send exception is ill-formed
2. don't unnecessarily reconstruct

AmeBel/atomspace,misgeatgit/atomspace,rTreutlein/atomspace,rTreutlein/atomspace,AmeBel/atomspace,rTreutlein/atomspace,AmeBel/atomspace,AmeBel/atomspace,rTreutlein/atomspace,AmeBel/atomspace,rTreutlein/atomspace,misgeatgit/atomspace,misgeatgit/atomspace,misgeatgit/atomspace,misgeatgit/atomspace