yield error if Command.workingDirectory doesn't exist

Otherwise some command might clutter the user's current
working dir.

Task-number: QBS-607
Change-Id: Id263481d1a615b9c66753e11b271441d4bd004b1
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>
Merge remote-tracking branch 'origin/1.2'

Change-Id: I603430421156e4d23451a7a4ba0f884d6556f89a