Indicate when response filter (JSONPath/XPath) input is invalid/non-existent

[yozlet]

Given that the response filter is evaluated on every keypress, it should be possible to tell whether the JSONPath/XPath/whatever expression is in one of these three states while typing:

1. Incorrect/unprocessable syntax
2. Syntactically valid, but broken for the current document (e.g. addressing a path that doesn't exist in the doc, so returns [] or similar)
3. Useful (returns any nodes at all)

For my uses, I see states 1 & 2 as effectively identical in that I need to fix the filter input. Right now, the only indication that I'm in either state is that the Response Preview just shows [] or similar, and that's bad for the reasons discussed in #1259.

It would be great to have a a visual indicator or decoration for these states (e.g. changing text colour, or adding a red dotted underline, or changing the background colour), as well as making that state queryable by the rest of the app (for #1259, maybe others).

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[gschier]

I think it would definitely help to show syntax errors 👍