Detect when a linter is not installed

[xelaadryth]

If a linter is not installed and you don't run with --install or --autorun, then we don't install the linter and the run will fail with an unhelpful error. For instance, with just jsonlint enabled but not installed:

> inline-plz --dryrun
Invalid message: The system cannot find the path specified.

If we add a "detect installation location" field to each linter to see if they're installed, we can check for file or directory presence and throw an exception instead of attempting to subprocess run a non-existent executable that gives better clarity on why it's failing.