Arcanist does not aggregate exceptions pertaining to missing external linters. We have four external linters configured (ArcanistCSSLintLinter, ArcanistLesscLinter, ArcanistJSHintLinter and ArcanistJSONLintLinter) and new devs end up doing something like this:

1. Writing some code.
2. arc diff
3. Arcanist throws an exception because csslint is not installed.
4. sudo npm install -g csslint
5. arc diff
6. Arcanist throws an exception because lessc is not installed.
7. sudo npm install -g less
8. arc diff
9. Arcanist throws an exception because jshint is not installed.
10. sudo npm install -g jshint
11. arc diff
12. Arcanist throws an exception because jsonlint is not installed.
13. sudo npm install -g jsonlint
14. arc diff (finally...)

This process is pretty painful and could be handled a lot better by allowing these exceptions to aggregate and just throwing a PhutilAggregateException instead. Probably we should define a new subclass of ArcanistUsageException for ArcanistExternalLinterException to differentiate these types of exceptions.