Allow configuration to be explicitly validated, including validation of complex attributes

Summary:
  - Allows us to implement setup warnings for edits which don't go through the web UI, e.g. "you edited a config file and set value X to something goofy".
  - Allows us to implement more sophisticated validations, beyond basic type checks (e.g., "phabricator.base-uri" must be a URI).
  - Fixes T358 (or, close enough -- fixes it for all options which have been migrated as per T2255.

Test Plan: Set "darkconsole.enabled" to "xyz" in my config, observed setup warning. Added fake validation, observed web UI edit error.

Reviewers: codeblock, btrahan

Reviewed By: codeblock

CC: aran

Maniphest Tasks: T2255, T358

Differential Revision: https://secure.phabricator.com/D4315

src/applications/config/controller/PhabricatorConfigEditController.php

@@ -78,10 +78,15 @@ final class PhabricatorConfigEditController
               PhabricatorContentSource::SOURCE_WEB,
               array(
                 'ip' => $request->getRemoteAddr(),
-              )))
-          ->applyTransactions($config_entry, array($xaction));
+              )));
 
-        return id(new AphrontRedirectResponse())->setURI($done_uri);
+        try {
+          $editor->applyTransactions($config_entry, array($xaction));
+          return id(new AphrontRedirectResponse())->setURI($done_uri);
+        } catch (PhabricatorConfigValidationException $ex) {
+          $e_value = pht('Invalid');
+          $errors[] = $ex->getMessage();
+        }
       }
     } else {
       $display_value = $this->getDisplayValue($option, $config_entry);