depri: Check for unprioritized task and print message.

The existing check didn't do much good, and one could deprioritize any normal task without notice. Instead, check that the task to be deprioritized actually has a priority and alert via "TODO: 42 is not prioritized." if that is not the case.

tests/t1700-depri.sh

@@ -68,4 +68,30 @@ TODO: 2 deprioritized.
 TODO: 3 of 3 tasks shown
 EOF
 
+cat > todo.txt <<EOF
+(B) smell the uppercase Roses +flowers @outside
+(A) notice the sunflowers
+stop
+EOF
+test_todo_session 'depriority of unprioritized task' <<EOF
+>>> todo.sh -p list
+2 (A) notice the sunflowers
+1 (B) smell the uppercase Roses +flowers @outside
+3 stop
+--
+TODO: 3 of 3 tasks shown
+
+>>> todo.sh depri 3 2
+TODO: 3 is not prioritized.
+2 notice the sunflowers
+TODO: 2 deprioritized.
+
+>>> todo.sh -p list
+1 (B) smell the uppercase Roses +flowers @outside
+2 notice the sunflowers
+3 stop
+--
+TODO: 3 of 3 tasks shown
+EOF
+
 test_done