Handle missing files when using the "Show Resulting File" button

[DavidStirling]

Fixes #59

Two possible scenarios to handle here:

• If the user had enabled the "split planes" options in raw2ometiff the "official" output file may not exist, triggering an error when pressing the "show me the resulting file" button.
• If the user deletes the resulting file for whatever reason and then hits the button, they'd get the same exception.

To handle this we now check if the output file exists, try the parent folder if it doesn't (case 1), and abort with a cleaner error message if even that doesn't exist.

[sbesson]

Tested the two following scenarios:

• conversion of a multi-plane file as a single file OME-TIFF followed by a deletion of the file and clicking the Show resulting file button
• conversion of a multi-plane file as using the Split planes options followed by clicking the Show resulting file button

Confirmed that both workflows trigger an UI error with the current HEAD of NGFF-Converter. With this PR included, both workflows open the parent directory.

If I also delete the parent directory, no action seems to happen but a Unable to locate output file status message is displayed in the status bar at the bottom of the application window.