appiler: do not keep applier diag in applier fiber diag

Currently last applier error is keeped in fiber diag. This leads to some
hacks like the return of -1 from applier fiber to keep the diag (fiber
diag is cleared if fiber return value is 0). Also we do not use applier
fiber diag as it intended - as diag set to current fiber on applier fiber
join. Let's instead keep applier diag in a different place.

Also let's copy error in `applier_wait_for_state` so that fiber error is
not disappeared in case of bootstrap.

Part of #8423

NO_TEST=refactoring
NO_CHANGELOG=refactoring
NO_DOC=refactoring