fix(parser): warn instead of silently dropping tasks with invalid due dates

A malformed due: value (e.g. due:2024-13-99) left DueDate nil, causing
ToRemind to silently emit nothing for that task. Now ParseLine reports
an error so the problem surfaces on stderr instead of vanishing.

Also fixes staticcheck QF1012 (WriteString(Sprintf(...)) -> Fprintf)
flagged by the IDE in ToRemind.

test.todo.txt

@@ -73,6 +73,9 @@ X 2012-01-01 Make resolutions
 (A) 2024-13-99 Call Mom
 x 2024-99-99 Call Mom
 
+# Malformed: due date with invalid format (should error, task dropped from output)
+(A) Call Mom due:2024-13-99
+
 # Malformed: key:value with whitespace in key or value
 foo bar:baz
 foo:bar baz