Fix get_icon

Property reply functions are now notified if there was an error with the
property instead of silent failure. Also issues in `get_item` were
resolved.

include/swaybar/tray/dbus.h

@@ -5,6 +5,13 @@
 #include <dbus/dbus.h>
 extern DBusConnection *conn;
 
+enum property_status {
+	PROP_EXISTS, /* Will give iter */
+	PROP_ERROR, /* Will not give iter */
+	PROP_BAD_DATA, /* Will not give iter */
+	PROP_WRONG_SIG, /* Will give iter, please be careful */
+};
+
 /**
  * Checks the signature of the given iter against `sig`. Prefer to
  * `dbus_message_iter_get_signature` as this one frees the intermediate string.
@@ -15,8 +22,9 @@ bool dbus_message_iter_check_signature(DBusMessageIter *iter, const char *sig);
  * Fetches the property and calls `callback` with a message iter pointing it.
  * Performs error handling and signature checking.
  *
- * Returns: true if message is successfully sent (will not necessarily arrive)
- * and false otherwise
+ * Returns: true if message is successfully sent and false otherwise. If there
+ * is an error getting a property, `callback` will still be run, but with
+ * `status` set to the error.
  *
  * NOTE: `expected_signature` must remain valid until the message reply is
  * received, please only use 'static signatures.
@@ -26,7 +34,7 @@ bool dbus_get_prop_async(const char *destination,
 		const char *iface,
 		const char *prop,
 		const char *expected_signature,
-		void(*callback)(DBusMessageIter *iter, void *data),
+		void(*callback)(DBusMessageIter *iter, void *data, enum property_status status),
 		void *data);
 /**
  * Should be called in main loop to dispatch events