Handle execvp error in open_browser

The execvp function usually doesn't return and exits by itself. But,
when having an error, it returns. As a faulty browser setting could
trigger this, dunst has to handle this, as the forked child wants to
access the X11 server too, resulting in XIO errors.

Fixes #476

src/menu.c

@@ -119,6 +119,12 @@ void open_browser(const char *in)
                             string_append(settings.browser, url, " ");
                         char **cmd = g_strsplit(browser_cmd, " ", 0);
                         execvp(cmd[0], cmd);
+                        // execvp won't return if it's successful
+                        // so, if we're here, it's definitely an error
+                        fprintf(stderr, "Warning: failed to execute '%s': %s\n",
+                                        settings.browser,
+                                        strerror(errno));
+                        exit(EXIT_FAILURE);
                 }
         }
 }