Treating only MDB_NOTFOUND as an error leads to use of uninitliazed
pointers and handle IDs in other cases (e.g. when get fails with
MDB_BAD_TXN) and wreaks havoc in the application.
CCBUG: 361186
CCBUG: 390823
CCBUG: 372880
CCBUG: 395888
CCBUG: 367480
CCBUG: 403720