Saving gmap_markers.js needs catch errors

This one is part of META issue #1931138: Fixing all 'single marker' issues

Around gmap.module, line 555, i get warnings sometimes cause permissions are wrong on gmap_markers.js file. When that occurs, $file is false, so accessing $file->fid is wrong. May be is necessary to catch that kind of error and show a better error message. For example:

// Make sure js/ exists in the files folder.
if (file_prepare_directory($dir, FILE_CREATE_DIRECTORY)) {
$file = file_save_data($contents, 'public://js/gmap_markers.js', FILE_EXISTS_REPLACE);
if ($file === false)
drupal_set_message(t('GMap is unable to save the marker bundle. Please check gmap_markers.js permissions!'), 'error');
else
variable_set('gmap_marker_file', $file->fid);
}
else {
drupal_set_message(t('GMap is unable to save the marker bundle. Markers will not work!'), 'error');
}