Exit on leveldb error

When the valhalla cluster goes sad, we see error messages which seem to imply that the leveldb cache is corrupt (IO Error, 5), but really isn't and can be fixed on remount. However, often, once this condition is hit, the mount stops responding appropriately to requests, but doesn't signal itself as down, so we have no indication that it is not working. With this fix, when we detect these leveldb errors, we will send a SIGTERM and let the fusedav process exit. Unfortunately, once it exits, the mount/automount get in a funny state where accesses to the files directory, instead of having automount remount, return a Too Many Symbolic Links error. That is an issue which still needs addressing, but might require lower-level investigation into systemd and kernel stuff.