plugins: Consistent error handling on FUA

plugins: Consistent error handling on FUA

We document that a plugin callback should return 0 on success, but
other places in the code treat all values other than -1 as success
(perhaps we should treat all negative values as errors, instead of
exactly -1, but that may break binary back-compatibility).  However,
when reworking where FUA fallback occurs, we introduced a subtle
change: if a plugin returns a non-zero value on success despite the
documentation, and the client requested FUA, we ended up reporting
success to the client without performing FUA.

Fixes: 4f37c64ffdd42fab5c5d9c6157db396b60866a7a
Signed-off-by: Eric Blake <eblake@redhat.com>