bisect: simplify return code from bisect_checkout()

bisect: simplify return code from bisect_checkout()

The function was designed to return only BISECT_OK (0) or
BISECT_FAILED (-1) and no other values, but there were two issues:

 - The comment misspelled BISECT_FAILED as BISECT_FAILURE, even
   though the logic it described (i.e. any non-zero return should be
   reported as a single BISECT_FAILED) was correct.

 - It took the return value from run_command_v_opt(), and assumed it
   was either -1 or 1 upon error, which is not the case; it can relay
   errors from wait_or_whine(), which can report exit status of the
   child process.

Translate any error return from run_command_v_opt() to BISECT_FAILED,
and simplify the resulting code by losing the 'res' variable that is
no longer needed.

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