Add warning when web extension could not be found.

The warning is only shown if compiled with DEBUG. Hope this will helps
users and package managers to check the compilation or installation.

src/main.c

@@ -1011,6 +1011,14 @@ static void on_webctx_init_web_extension(WebKitWebContext *webctx, gpointer data
     const char *name;
     GVariant *vdata;
 
+#ifdef DEBUG
+    char *extension = g_build_filename(EXTENSIONDIR,  "webext_main.so", NULL);
+    if (!g_file_test(extension, G_FILE_TEST_IS_REGULAR)) {
+        g_warning("Cannot access web extension %s", extension);
+    }
+    g_free(extension);
+#endif
+
     /* Setup the extension directory. */
     webkit_web_context_set_web_extensions_directory(webctx, EXTENSIONDIR);