Linter issues

[DonJayamanne]

• When checking whether a linter exists, we're always treating linters as modules in the current python installation. I.e. if user has provided fully qualified path to linter, we still check if linter is installed using pythong -c 'import <linter>'
• The telemetry captured to determine whether a user has provided a fully qualified path to a linter no longer works (logic is incorrect)
• Assume user enters the path to a linter in settings.json, and the path is invalid.
  We'll display the message Linter xxx is not installed [Install] [Disable xxx] [Disable linting]
  If user opts to install, they'll keep getting the above error as the path provided in settings.json is invalid.

[DonJayamanne]

Sample messages that will be displayed for Test Frameworks, Linters and Formatters (when the paths are invalid):

[brettcannon]

The prompt is not triggered if you specify "doesnotexist"; it seems to be specific to lacking a path separator.

[DonJayamanne]

@brettcannon
Do you not get any prompt when you use doesnotexist?
There are two types of prompts, one that will mention the invalid path and the other thats a generic message.
Works for me on both Mac and Windows.

[brettcannon]

I get the generic message to install Black.