The TDB1 code's tdb1_find() returns 0 on error; the callers should
not assume that the error means that the entry wasn't found, but use
last_error to determine it.
This was found by looking at how long the failure path testing for
test/run-10-simple-store.c was taking under valgrind, ie:
valgrind -q ./run-10-simple-store --show-slowest
This change dropped the time for that test from 53 seconds to 19
seconds.