Check if directories are valid.

Things to test:

On startup:

• do the paths set in the configuration / status file exist after all
  • create if missing
    • incoming
    • managed, including PROCESSING, UNMATCHED and DONE
  • do NOT create if missing, refuse to start and log a message
    • DestinationDirectory
    • TmpTransferDir
  • do NOT create if missing, log a message and reset the values
    • unfinished transfer source CurrentTransferSrc
    • unfinished transfer temp CurrentTargetTmp

When new data is found in INCOMING:

• does the "owner" directory correspond to an ActiveDirectory user name (covered by the next point, as we only have folders on the remote share for users from the AD)
• does the corresponding folder exist on the remote share

If one of these fails, send an admin notification and don't process the data (move to UNMATCHED or similar).

See also issue #10 (closed).