I understand the issue. I don't recall why the GBC developers chose not to render ERROR/MESSAGE same as GDC and respect the values in the .4st with respect to the position, or add a new style attribute value for the rendering they have chosen.
You should note that the user can always tap on the close icon in the error/message to make it disappear and read what has been obscured.
There are some theme variables you can use to alter the behaviour via GBC customisation.
Look in
http://4js.com/online_documentation/fjs-gbc-manual-html/reference/theme/theme_variables.htmlfor theme-message-display-position, theme-message-display-time, theme-error-display-time, theme-error-display-position
You may decide that changing the position may lessen the chance of something important being obscured
or decide to change the time values so that the error/message fades out after X seconds.
if you want to do more advanced customisation then grepping for those keywords in the GBC customisation sources should give you a good place to start
Reuben