improve GPX parsing error message

[ptpt]

A more explicit error when GPX parsing fails:

[
  {
    "error": {
      "type": "RuntimeError",
      "message": "Error parsing GPX /usr/xx/Downloads/Forward.gpx: UnicodeDecodeError: 'utf-8' codec can't decode byte 0xb0 in position 37: invalid start byte"
    },
    "filename": "/usr/xx/Downloads/Forward.mp4",
    "filetype": "video"
  }
]