[ui] Reduce confusion when qml loading fails

Currently we disable all logging out of qml by default. This is
problematic in case qml loading fails for any reason (e.g. missing
dependencies) as the user will be presented with a message that is not
actionable: "QQmlApplicationEngine failed to load component". There is
no way to understand what's happening unless the user knows about
MESHROOM_OUTPUT_QML_WARNINGS.

This is improved by checking for presence of "QQmlApplicationEngine
failed to load component" and warning if qml logging is disabled in that
case.

meshroom/ui/app.py

@@ -51,9 +51,19 @@ class MessageHandler(object):
     @classmethod
     def handler(cls, messageType, context, message):
         """ Message handler remapping Qt logs to Python logging system. """
-        # discard blacklisted Qt messages related to QML when 'output qml warnings' is set to false
-        if not cls.outputQmlWarnings and any(w in message for w in cls.qmlWarningsBlacklist):
-            return
+
+        if not cls.outputQmlWarnings:
+            # If MESHROOM_OUTPUT_QML_WARNINGS is not set and an error in qml files happen we're
+            # left without any output except "QQmlApplicationEngine failed to load component".
+            # This is extremely hard to debug to someone who does not know about
+            # MESHROOM_OUTPUT_QML_WARNINGS beforehand because by default Qml will output errors to
+            # stdout.
+            if "QQmlApplicationEngine failed to load component" in message:
+                logging.warning("Set MESHROOM_OUTPUT_QML_WARNINGS=1 to get a detailed error message.")
+
+            # discard blacklisted Qt messages related to QML when 'output qml warnings' is not enabled
+            elif any(w in message for w in cls.qmlWarningsBlacklist):
+                return
         MessageHandler.logFunctions[messageType](message)