[Qt] Expose mapLoadingFailed signal

platform/qt/include/qmapboxgl.hpp

@@ -251,6 +251,7 @@ public slots:
 signals:
     void needsRendering();
     void mapChanged(QMapboxGL::MapChange);
+    void mapLoadingFailed(const QString &reason);
     void copyrightsChanged(const QString &copyrightsHtml);
 
     void staticRenderFinished(const QString &error);

platform/qt/src/qmapboxgl.cpp

@@ -1649,6 +1649,7 @@ QMapboxGLPrivate::QMapboxGLPrivate(QMapboxGL *q, const QMapboxGLSettings &settin
     qRegisterMetaType<QMapboxGL::MapChange>("QMapboxGL::MapChange");
 
     connect(m_mapObserver.get(), SIGNAL(mapChanged(QMapboxGL::MapChange)), q, SIGNAL(mapChanged(QMapboxGL::MapChange)));
+    connect(m_mapObserver.get(), SIGNAL(mapLoadingFailed(QString)), q, SIGNAL(mapLoadingFailed(QString)));
     connect(m_mapObserver.get(), SIGNAL(copyrightsChanged(QString)), q, SIGNAL(copyrightsChanged(QString)));
 
     // Setup the Map object

platform/qt/src/qmapboxgl_map_observer.cpp

@@ -2,6 +2,10 @@
 
 #include "qmapboxgl_p.hpp"
 
+#include <mbgl/util/string.hpp>
+
+#include <exception>
+
 QMapboxGLMapObserver::QMapboxGLMapObserver(QMapboxGLPrivate *d)
     : d_ptr(d)
 {
@@ -44,9 +48,10 @@ void QMapboxGLMapObserver::onDidFinishLoadingMap()
     emit mapChanged(QMapboxGL::MapChangeDidFinishLoadingMap);
 }
 
-void QMapboxGLMapObserver::onDidFailLoadingMap(std::exception_ptr)
+void QMapboxGLMapObserver::onDidFailLoadingMap(std::exception_ptr exception)
 {
     emit mapChanged(QMapboxGL::MapChangeDidFailLoadingMap);
+    emit mapLoadingFailed(QString::fromStdString(mbgl::util::toString(exception)));
 }
 
 void QMapboxGLMapObserver::onWillStartRenderingFrame()

platform/qt/src/qmapboxgl_map_observer.hpp

@@ -36,6 +36,7 @@ public:
 
 signals:
     void mapChanged(QMapboxGL::MapChange);
+    void mapLoadingFailed(const QString &reason);
     void copyrightsChanged(const QString &copyrightsHtml);
 
 private: