Make omitting "value" from a transaction description an explicit error

Summary: See PHI1710. Until D21044, some transactions could omit "value" and apply correctly. This now throws an exception when accessing `$xaction['value']`. All transactions are expected to have a "value" key, so require it explicitly rather than implicitly.

Test Plan: Submitted a transaction with a "type" but no "value". After D21044, got a language-level exception. After this change, got an explicit exception.

Differential Revision: https://secure.phabricator.com/D21176

src/applications/transactions/editengine/PhabricatorEditEngine.php

@@ -2204,6 +2204,16 @@ abstract class PhabricatorEditEngine
             $transactions_key,
             $key));
       }
+
+      if (!array_key_exists('value', $xaction)) {
+        throw new Exception(
+          pht(
+            'Parameter "%s" must contain a list of transaction descriptions, '.
+            'but item with key "%s" is missing a "value" field. Each '.
+            'transaction must have a value field.',
+            $transactions_key,
+            $key));
+      }
     }
 
     return $xactions;