Bug 1373368 - [mozlint] Raise if a non-existant path is specified in a lint config, r=standard8

Since I left the next two patches to bitrot, I realized that a path I had added
didn't exist anymore. We should definitely error out if non-existant paths are
specified, otherwise the lists will become outdated and it will be possible to
accidentally disable linting on some files.

I discovered a few instances of this already in our existing definitions.

MozReview-Commit-ID: 8jsTKLI0nFE

python/mozlint/test/test_parser.py

@@ -15,10 +15,12 @@ from mozlint.errors import (
     LinterParseError,
 )
 
+here = os.path.abspath(os.path.dirname(__file__))
+
 
 @pytest.fixture(scope='module')
 def parse(lintdir):
-    parser = Parser()
+    parser = Parser(here)
 
     def _parse(name):
         path = os.path.join(lintdir, name)
@@ -48,6 +50,9 @@ def test_parse_valid_linter(parse):
     'invalid_exclude.yml',
     'missing_attrs.yml',
     'missing_definition.yml',
+    'non_existing_include.yml',
+    'non_existing_exclude.yml',
+    'non_existing_support_files.yml',
 ])
 def test_parse_invalid_linter(parse, linter):
     with pytest.raises(LinterParseError):