Handle error from fopen_path in finish_recovery

The fopen_path will fail here e.g. if /cache is corrupt and could
not be mounted. Not properly handling an error code from
fopen_path() results in a boot loop into recovery as fwrite() on
a NULL FILE object will crash the recovery app.

Change-Id: I1ccff5a8cb620a32f32adfe97a3fae8e5e908f39