Feature - #310 'task add' with external editor

- Simplified and make clearer and error message that complained about
  things that were beyond user control (thanks to John Florian).
(cherry picked from commit a2152628251c6d8c9bc840b8f36851f4ce680c99)

Signed-off-by: Paul Beckingham <paul@beckingham.net>

src/Task.cpp

@@ -527,7 +527,7 @@ void Task::validate () const
   if (!has ("uuid")  ||
       !has ("entry") ||
       !has ("description"))
-    throw std::string ("A task must have a uuid, entry date and description in order to be valid."); // TODO i18n
+    throw std::string ("A task must have a description in order to be valid."); // TODO i18n
 
   if (get ("description") == "") // No i18n
     throw std::string ("Cannot add a task that is blank, or contains <CR> or <LF> characters."); // TODO i18n