mrview Screenshot tool: Fix dir path handling
mrview Screenshot tool: Fix dir path handling

Make sure that the output directory path is always valid, by aborting the function immediately if the user cancels in the file dialog.
Closes #393.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3