Fix empty message for certain tracker status errors

By design alert.msg will be empty if the error code is '-1' so use
a.e.message() to get the message as fallback. It was not used at
replacement because when error code is not '-1' then a.e.message()
will also include the error code, which we do not want.
Fixed glade missing signal warning in path_combo_chooser
add comment to tracker_error_alert handler