Error( messages... )
Error signals  an  error.  First  the messages messages are  printed,
this  is done exactly as if Print (see Print) were  called with these
arguments.  Then a break loop (see Break Loops) is  entered, unless the
standard error output is not connected to a terminal.  You can leave this
break  loop  with return;  to  continue execution  with  the  statement
following the call to Error.
GAP 3.4.4