bpo-34248: Add filename to error raised in {gnu,ndbm}.open() (GH-8590)

bpo-34248: Add filename to error raised in {gnu,ndbm}.open() (GH-8590)

Report the filename to the exception when raising {gdbm,dbm.ndbm}.error in
dbm.gnu.open() and dbm.ndbm.open() functions, so it gets printed when the
exception is raised, and can also be obtained by the filename attribute of the
exception object.