aft: Unify handling of hash and path duplicates.

These are symmetric but were not treated as such. Specifically:

	* We ignored fatal errors for finding hash sisters while path
	brothers were handled strictly.

	* For hash sisters, we had a helper which implicitly special
	cased the non-fatal E_OSL_RB_KEY_NOT_FOUND case while the
	same check was open-coded for path brothers.

This adds the find_path_brother() helper which is analogous to
find_hash_sister() and fixes up the latter function to perform strict
error checking rather than relying on the implementation detail that
osl_get_row() sets the result pointer to NULL if no entry was found.