Browser: Show a message when attempting to search with no search engine

Previously, it just showed an error message saying that "" failed to
load, which made me think that search engines were broken.

Userland/Applications/Browser/Tab.cpp

@@ -21,6 +21,7 @@
 #include <LibGUI/Button.h>
 #include <LibGUI/Clipboard.h>
 #include <LibGUI/Menu.h>
+#include <LibGUI/MessageBox.h>
 #include <LibGUI/Statusbar.h>
 #include <LibGUI/TextBox.h>
 #include <LibGUI/Toolbar.h>
@@ -145,6 +146,11 @@ Tab::Tab(BrowserWindow& window, Type type)
     m_location_box->set_placeholder("Address");
 
     m_location_box->on_return_pressed = [this] {
+        if (m_location_box->text().starts_with('?') && g_search_engine.is_empty()) {
+            GUI::MessageBox::show(&this->window(), "Select a search engine in the Settings menu before searching.", "No search engine selected", GUI::MessageBox::Type::Information);
+            return;
+        }
+
         auto url = url_from_user_input(m_location_box->text());
         load(url);
         view().set_focus(true);