Added a static method for concise condition requirey in invocation services.

git-svn-id: svn+ssh://src.earth.threerings.net/narya/trunk@6583 542714f4-19e9-0310-aa3c-eee0fc999fb1

src/main/java/com/threerings/presents/server/InvocationException.java

@@ -54,6 +54,18 @@ public class InvocationException extends Exception
         requireAccess(clobj, perm, null);
     }
 
+    /**
+     * Requires that the supplied condition be true, otherwise an invocation exception with the
+     * supplied error message is thrown.
+     */
+    public static void require (boolean condition, String errmsg)
+        throws InvocationException
+    {
+        if (!condition) {
+            throw new InvocationException(errmsg);
+        }
+    }
+
     /**
      * Constructs an invocation exception with the supplied cause code string.
      */