Improve behavior for detecting the failure to parse external linter output

Summary:
Currently, a lot of `ArcanistExternalLinter` subclasses have something like this:

```lang=php
if ($err && !$messages) {
  return false;
}
```

We can avoid this code duplication by moving this check to the `ArcanistExternalLinter` class.

Test Plan: `arc unit`

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11321

src/lint/linter/ArcanistCoffeeLintLinter.php

@@ -120,10 +120,6 @@ final class ArcanistCoffeeLintLinter extends ArcanistExternalLinter {
       }
     }
 
-    if ($err && !$messages) {
-      return false;
-    }
-
     return $messages;
   }
 

src/lint/linter/ArcanistCpplintLinter.php

@@ -56,10 +56,6 @@ final class ArcanistCpplintLinter extends ArcanistExternalLinter {
       $messages[] = $message;
     }
 
-    if ($err && !$messages) {
-      return false;
-    }
-
     return $messages;
   }
 

src/lint/linter/ArcanistExternalLinter.php

@@ -354,6 +354,13 @@ abstract class ArcanistExternalLinter extends ArcanistFutureLinter {
 
     $messages = $this->parseLinterOutput($path, $err, $stdout, $stderr);
 
+    if ($err && $this->shouldExpectCommandErrors() && !$messages) {
+      // We assume that if the future exits with a non-zero status and we
+      // failed to parse any linter messages, then something must've gone wrong
+      // during parsing.
+      $messages = false;
+    }
+
     if ($messages === false) {
       if ($err) {
         $future->resolvex();

src/lint/linter/ArcanistFlake8Linter.php

@@ -78,10 +78,6 @@ final class ArcanistFlake8Linter extends ArcanistExternalLinter {
       $messages[] = $message;
     }
 
-    if ($err && !$messages) {
-      return false;
-    }
-
     return $messages;
   }
 

src/lint/linter/ArcanistJSONLintLinter.php

@@ -79,10 +79,6 @@ final class ArcanistJSONLintLinter extends ArcanistExternalLinter {
       }
     }
 
-    if ($err && !$messages) {
-      return false;
-    }
-
     return $messages;
   }
 

src/lint/linter/ArcanistJscsLinter.php

@@ -129,10 +129,6 @@ final class ArcanistJscsLinter extends ArcanistExternalLinter {
       }
     }
 
-    if ($err && !$messages) {
-      return false;
-    }
-
     return $messages;
   }
 

src/lint/linter/ArcanistLesscLinter.php

@@ -177,10 +177,6 @@ final class ArcanistLesscLinter extends ArcanistExternalLinter {
       }
     }
 
-    if ($err && !$messages) {
-      return false;
-    }
-
     return $messages;
   }
 

src/lint/linter/ArcanistPEP8Linter.php

@@ -70,10 +70,6 @@ final class ArcanistPEP8Linter extends ArcanistExternalLinter {
       $messages[] = $message;
     }
 
-    if ($err && !$messages) {
-      return false;
-    }
-
     return $messages;
   }
 

src/lint/linter/ArcanistPuppetLintLinter.php

@@ -133,10 +133,6 @@ final class ArcanistPuppetLintLinter extends ArcanistExternalLinter {
       $messages[] = $message;
     }
 
-    if ($err && !$messages) {
-      return false;
-    }
-
     return $messages;
   }
 

src/lint/linter/ArcanistPyFlakesLinter.php

@@ -77,10 +77,6 @@ final class ArcanistPyFlakesLinter extends ArcanistExternalLinter {
       $messages[] = $message;
     }
 
-    if ($err && !$messages) {
-      return false;
-    }
-
     return $messages;
   }
 

src/lint/linter/ArcanistPyLintLinter.php

@@ -142,10 +142,6 @@ final class ArcanistPyLintLinter extends ArcanistExternalLinter {
       $messages[] = $message;
     }
 
-    if ($err && !$messages) {
-      return false;
-    }
-
     return $messages;
   }
 

src/lint/linter/ArcanistRubyLinter.php

@@ -81,10 +81,6 @@ final class ArcanistRubyLinter extends ArcanistExternalLinter {
       $messages[] = $message;
     }
 
-    if ($err && !$messages) {
-      return false;
-    }
-
     return $messages;
   }