Reject invalid match criteria with an error.

Previously, using a command like

  [con_id=foo] kill

would kill the currently focused window because while an error for
not being able to parse the con_id was logged, no further action
was taken, which caused the criterion to be ignored. In this case,
the fallback behavior of using the focused window took over.

For con_id, id and window_type we now reject incorrect values with
an error and abort the command.

fixes #2091

include/data.h

@@ -436,6 +436,9 @@ struct Window {
  *
  */
 struct Match {
+    /* Set if a criterion was specified incorrectly. */
+    char *error;
+
     struct regex *title;
     struct regex *application;
     struct regex *class;