extensionSystem: Catch import errors

While we catch errors that occur when calling init(), enable() or
disable(), the import itself can throw an exception, for instance
if the extension imports an unavailable typelib or tries to draw
in a conflicting library.

https://bugzilla.gnome.org/show_bug.cgi?id=781728

js/ui/extensionSystem.js

@@ -225,7 +225,12 @@ function initExtension(uuid) {
     let extensionState = null;
 
     ExtensionUtils.installImporter(extension);
-    extensionModule = extension.imports.extension;
+    try {
+        extensionModule = extension.imports.extension;
+    } catch(e) {
+        logExtensionError(uuid, e);
+        return false;
+    }
 
     if (extensionModule.init) {
         try {