refs/files: improve error handling when verifying symrefs

refs/files: improve error handling when verifying symrefs

The error handling when verifying symbolic refs is a bit on the wild
side:

  - `fsck_report_ref()` can be told to ignore specific errors. If an
    error has been ignored and a previous check raised an unignored
    error, then assigning `ret = fsck_report_ref()` will cause us to
    swallow the previous error.

  - When the target reference is not valid we bail out early without
    checking for other errors.

Fix both of these issues by consistently or'ing the return value and not
bailing out early.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>