--validate should print JSON if files don't exist

```
$ elm-format --validate xxx
[There was a problem reading one or more of the specified input files:

xxx: File does not exist

Please check the given paths.
```
Should instead print:
```
$ elm-format --validate xxx
[{"path":"xxx","message":"File does not exist"}
]
```

Start by tracing the elm-format --validate command entry point and how it handles a nonexistent path such as xxx. Run the command before and after the change, and confirm the result is JSON containing the path and the “File does not exist” message.

Written by the indexing model from the issue text.