traverse_trees_recursive(): propagate merge errors up

traverse_trees_recursive(): propagate merge errors up

There were few places where merge errors detected deeper in the call chain
were ignored and not propagated up the callchain to the caller.

Most notably, this caused switching branches with "git checkout" to ignore
a path modified in a work tree are different between the HEAD version and
the commit being switched to, which it internally notices but ignores it,
resulting in an incorrect two-way merge and loss of the change in the work
tree.

Signed-off-by: Junio C Hamano <gitster@pobox.com>