Avoid crash when no app can open an external article link

[jim-daf]

What

When an article links to an external URL, ArticleWebView.shouldOverrideUrlLoading hands it off with a raw getContext().startActivity(new Intent(Intent.ACTION_VIEW, uri)). On a device with no app able to handle the URI that call throws ActivityNotFoundException and the article view crashes.

This wraps the launch in a try/catch and shows a short toast when nothing can open the link, mirroring the existing msg_no_activity_to_get_content handling for the open-file case. Links that resolve behave exactly as before.

Changes

                 if (isExternal(uri)) {
                     Intent browserIntent = new Intent(Intent.ACTION_VIEW, uri);
-                    getContext().startActivity(browserIntent);
+                    try {
+                        getContext().startActivity(browserIntent);
+                    } catch (ActivityNotFoundException e) {
+                        Toast.makeText(getContext(), R.string.msg_no_activity_to_open_link,
+                                Toast.LENGTH_SHORT).show();
+                    }
                     return true;
                 }

A msg_no_activity_to_open_link string is added next to the existing msg_no_activity_to_get_content entry.

Closes #213

[jim-daf]

Closing this, the external link guard is already handled in #211. Sorry for the duplicate.