A few fixups in error handling: mark pg_re_throw() as noreturn for gcc,

A few fixups in error handling: mark pg_re_throw() as noreturn for gcc,
and for other compilers, insert a dummy exit() call so that they understand
PG_RE_THROW() doesn't return.  Insert fflush(stderr) in ExceptionalCondition,
per recent buildfarm evidence that that might not happen automatically on some
platforms.  And const-ify ExceptionalCondition's declaration while at it.