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

python/samba/mdb_util.py

@@ -19,6 +19,7 @@
 import samba
 import subprocess
 import os
+from samba.netcmd import CommandError
 
 
 def mdb_copy(file1, file2):
@@ -26,11 +27,17 @@ def mdb_copy(file1, file2):
     """
     # Find the location of the mdb_copy tool
     dirs = os.getenv('PATH').split(os.pathsep)
+    found = False
     for d in dirs:
         toolpath = os.path.join(d, "mdb_copy")
         if os.path.exists(toolpath):
+            found = True
             break
 
+    if not found:
+        raise CommandError("mdb_copy not found. "
+                           "You may need to install the lmdb-utils package")
+
     mdb_copy_cmd = [toolpath, "-n", file1, "%s.copy.mdb" % file1]
     status = subprocess.check_call(mdb_copy_cmd, close_fds=True, shell=False)