Give "bin/config" a friendlier error message if "local.json" is not writable

Summary: Ref T13403. We currently emit a useful error message, but it's not tailored and has a stack trace. Since this is a relatively routine error and on the first-time-setup path, tailor it so it's a bit nicer.

Test Plan:
  - Ran `bin/config set ...` with an unwritable "local.json".
  - Ran `bin/config set ...` normally.

Maniphest Tasks: T13403

Differential Revision: https://secure.phabricator.com/D20779

src/applications/config/management/PhabricatorConfigManagementSetWorkflow.php

@@ -140,11 +140,22 @@ final class PhabricatorConfigManagementSetWorkflow
         'Wrote configuration key "%s" to database storage.',
         $key);
     } else {
-      $config_source = id(new PhabricatorConfigLocalSource())
+      $config_source = new PhabricatorConfigLocalSource();
-        ->setKeys(array($key => $value));
 
       $local_path = $config_source->getReadablePath();
 
+      try {
+        Filesystem::assertWritable($local_path);
+      } catch (FilesystemException $ex) {
+        throw new PhutilArgumentUsageException(
+          pht(
+            'Local path "%s" is not writable. This file must be writable '.
+            'so that "bin/config" can store configuration.',
+            Filesystem::readablePath($local_path)));
+      }
+
+      $config_source->setKeys(array($key => $value));
+
       $write_message = pht(
         'Wrote configuration key "%s" to local storage (in file "%s").',
         $key,