mdb_util: Better error message if lmdb-utils not installed

mdb_util: Better error message if lmdb-utils not installed

mdb_copy() was dutifully checking the PATH for the mdb_copy executable,
then, if it didn't find it, blindly proceeding anyway and trying to run
a non-existent executable. This resulted in a cryptic error:

  ERROR(<type 'exceptions.OSError'>): uncaught exception - [Errno 2] No
    such file or directory

Add in an extra check that we actually find the executable and raise a
better human-readable exception if we don't.

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Reviewed-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Alexander Bokovoy <ab@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Fri Nov  9 21:07:47 CET 2018 on sn-devel-144