Catch ffmpeg export fails and display a nicer error message

at the moment we simply show "Export failed". Some more explanation might be nice (along the lines of "We failed to export your [audio/video]. This may happen because there was not enough free disk space or because you did not choose a file format that ffmpeg understands. You can find the full report in the file [filename]. If you think this should not happen, please open an issue or write us an email"

No file or test is named. Start by locating the ffmpeg export failure handling and the current "Export failed" message. Update the failure feedback with the requested explanation and report-file reference, then verify that an ffmpeg export failure displays the fuller message.

Written by the indexing model from the issue text.