Failing an LSM work unit should not cause the worker thread to exit.

At least for EBUSY and WT_NOTFOUND.

References #1185
lint
                  mod->mod_multi_entries > bnd_slot) {
btree/rec_write.c  2634  Warning 574: Signed-unsigned mix with relational
btree/rec_write.c  2634  Info 737: Loss of sign in promotion from int
to unsigned int