Ensure that in all flex lexers that are part of the backend, a

yy_fatal_error() call results in elog(ERROR) not exit().  This was
already fixed in the main lexer and plpgsql, but extend same technique
to all the other dot-l files.  Also, on review of the possible calls
to yy_fatal_error(), it seems safe to use elog(ERROR) not elog(FATAL).