Platform: prefix error messages coming from SDL2 and GLFW.

So it's clear the error comes from Magnum and not something else. Also
unify these and make GLFW not report the error on two lines if the init
fails.

Ugh, except that glfwGetError() is only there since 3.3, so I have to
keep the old code as well.

src/Magnum/Platform/GlfwApplication.cpp

@@ -95,10 +95,16 @@ GlfwApplication::GlfwApplication(const Arguments& arguments, NoCreateT):
         .parse(arguments.argc, arguments.argv);
     #endif
 
-    /* Init GLFW */
+    /* GLFW before 3.3 doesn't have glfwGetError(), use glfwSetErrorCallback()
+       even before glfwInit() in that case. It'll be on two lines but better
+       than getting no reason at all. */
+    #if GLFW_VERSION_MAJOR*100 + GLFW_VERSION_MINOR < 303
     glfwSetErrorCallback([](int, const char* const description) {
-        Error{} << description;
+        Error{} << "Platform::GlfwApplication:" << description;
     });
+    #endif
+
+    /* Init GLFW */
     #ifdef CORRADE_TARGET_APPLE
     /* Don't change current working directory to Resources/ in the app bundle
        on Apple platforms. Not sure why this would be done only on a single
@@ -108,10 +114,26 @@ GlfwApplication::GlfwApplication(const Arguments& arguments, NoCreateT):
     glfwInitHint(GLFW_COCOA_CHDIR_RESOURCES, false);
     #endif
     if(!glfwInit()) {
-        Error() << "Could not initialize GLFW";
+        /* On GLFW before 3.3 the error is printed by the callback that's set
+           up above */
+        #if GLFW_VERSION_MAJOR*100 + GLFW_VERSION_MINOR < 303
+        Error{} << "Platform::GlfwApplication: could not initialize GLFW";
+        #else
+        const char* error = nullptr;
+        CORRADE_INTERNAL_ASSERT_OUTPUT(glfwGetError(&error) != GLFW_NO_ERROR && error);
+        Error{} << "Platform::GlfwApplication: could not initialize GLFW:" << error;
+        #endif
         std::exit(8);
     }
 
+    /* Set error callback for further errors. On GLFW 3.3+ done after init so
+       we don't print the error message twice. */
+    #if GLFW_VERSION_MAJOR*100 + GLFW_VERSION_MINOR >= 303
+    glfwSetErrorCallback([](int, const char* const description) {
+        Error{} << "Platform::GlfwApplication:" << description;
+    });
+    #endif
+
     /* Save command-line arguments */
     if(args.value("log") == "verbose") _verboseLog = true;
     const Containers::StringView dpiScaling = args.value<Containers::StringView>("dpi-scaling");

src/Magnum/Platform/Sdl2Application.cpp

@@ -239,7 +239,7 @@ Sdl2Application::Sdl2Application(const Arguments& arguments, NoCreateT):
     #endif
 
     if(SDL_Init(SDL_INIT_VIDEO) < 0) {
-        Error() << "Cannot initialize SDL:" << SDL_GetError();
+        Error{} << "Platform::Sdl2Application: could not initialize SDL:" << SDL_GetError();
         std::exit(1);
     }