recovery: check return value of `fiber_new_system` for watcher fiber

`fiber_new_system` can potentially fail — its return value for the watcher
fiber must be checked and an exception must be raised in case it does fail.

Closes tarantool/security#87

NO_CHANGELOG=<security fix>
NO_DOC=<security fix>
NO_TEST=<no test harness for checking OOM>

(cherry picked from commit e9fad4c7)