prepare_revision_walk(): check for return value in all places

Even the documentation tells us:

	You should check if it returns any error (non-zero return
	code) and if it does not, you can start using get_revision()
	to do the iteration.

In preparation for this commit, I grepped all occurrences of
prepare_revision_walk and added error messages, when there were none.

Signed-off-by: Stefan Beller <stefanbeller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>