text: Title: Bad error message when using Rebot with a non-existing JSON output file
  Body: To reproduce:
  ```
  $ rebot non_existing.json
  [ ERROR ] Reading JSON source 'non_existing.json' failed: Loading JSON data failed: Invalid JSON data: JSONDecodeError: Expecting value: line 1 column 1 (char 0)
  
  Try --help for usage information.
  ```
  The error is much better when trying to use a non-existing XML output file:
  ```
  $ rebot non_existing.xml
  [ ERROR ] Reading XML source 'non_existing.xml' failed: No such file or directory
  
  Try --help for usage information.
  ```