Fix error message style and typo in exttab am code
Fix error message style and typo in exttab am code

Align a few error messages to match upstream coding convention for
error messages (which we are slowly adapting to), and fix a typo.
Spotted while fixing other things in the file.

Reviewed-by: Venkatesh Raghavan <2ac9b80e18a33e30d39310e3c14d26dc16fa3863@pivotal.io>

jmcatamney/gpdb,50wu/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,50wu/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,50wu/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,adam8157/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,50wu/gpdb,jmcatamney/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,ashwinstar/gpdb,lisakowen/gpdb,adam8157/gpdb,ashwinstar/gpdb,adam8157/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,xinzweb/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,jmcatamney/gpdb,adam8157/gpdb,ashwinstar/gpdb