Throw error if trying to set end date for a pending task

-throw error if end date is set for pending task
-tests

src/Task.cpp

@@ -1474,7 +1474,10 @@ void Task::validate(bool applyDefault /* = true */) {
     setAsNow("end");
 
   // Pending tasks cannot have an end date, remove if present
-  if ((status == Task::pending) && (get("end") != "")) remove("end");
+  if ((status == Task::pending) && (get("end") != "")) {
+    remove("end");
+    throw format("Could not modify task {1}. You cannot set an end date on a pending task.", this->id);
+  }
 
   // Provide a modified date unless user already specified one.
   if (!has("modified") || get("modified") == "") setAsNow("modified");

test/modify.test.py

@@ -55,6 +55,16 @@ def test_mod_nop(self):
         code, out, err = self.t("1 modify due:")
         self.assertIn("Modified 0 tasks.", out)
 
+class TestBug3584(TestCase):
+    def setUp(self):
+        self.t = Task()
+
+    def test_mod_pending_task_end_date(self):
+        """Adding the end date for a pending task throws an error"""
+        self.t("add foo")
+        code, out, err = self.t.runError("1 modify end:1d")
+        self.assertIn("Could not modify task 1. You cannot set an end date on a pending task.", err)
+
 
 if __name__ == "__main__":
     from simpletap import TAPTestRunner