Improve error message for invalid characters in output filenames

The fewer issues I have to read, the more new features I will have time to implement, so I ask that you please try these things first

• Check out all the menus and keyboard shortcuts. Maybe the feature already exists?
• Try with the newest version from GitHub
• Read the documentation and FAQ, Known issues, Troubleshooting to see if the feature already exists
• See if someone already suggested this under Issues or Discussions

Description

Please add a clearer error message when an export fails because the output filename contains invalid Windows characters such as ?, :, *, ", <, >, |, / or \.

Currently LosslessCut only shows "unable to export this file” error, which makes it look like there is a problem with the video or codec. A simple message like “The output filename contains an invalid character: ?” would save a lot of troubleshooting.