Avoid throwing away the error message in syncrep_yyerror.

Avoid throwing away the error message in syncrep_yyerror.

Commit 473a575e05979b4dbb28b3f2544f4ec8f184ce65 purported to make this
function stash the error message in *syncrep_parse_result_p, but
it didn't actually.

As a result, an attempt to set synchronous_standby_names to any value
that does not parse resulted in a generic "parser failed." message
rather than anything more specific. This fixes that.

Discussion: http://postgr.es/m/CA+TgmoYF9wPNZ-Q_EMfib_espgHycY-eX__6Tzo2GpYpVXqCdQ@mail.gmail.com
Backpatch-through: 18