gpcloud: check returned body before parsing
gpcloud: check returned body before parsing

Compatible S3 services don't always return error with XML body, check it
before parsing.

Signed-off-by: Peifeng Qiu <5ac57522f17d595d0e5c6d4a23fb4fdcd3f0c5e9@pivotal.io>

janebeckman/gpdb,50wu/gpdb,ashwinstar/gpdb,edespino/gpdb,xinzweb/gpdb,Quikling/gpdb,adam8157/gpdb,janebeckman/gpdb,greenplum-db/gpdb,adam8157/gpdb,Quikling/gpdb,cjcjameson/gpdb,xinzweb/gpdb,Chibin/gpdb,ashwinstar/gpdb,rvs/gpdb,cjcjameson/gpdb,lisakowen/gpdb,cjcjameson/gpdb,jmcatamney/gpdb,Chibin/gpdb,jmcatamney/gpdb,janebeckman/gpdb,rvs/gpdb,Chibin/gpdb,rvs/gpdb,Chibin/gpdb,rvs/gpdb,cjcjameson/gpdb,xinzweb/gpdb,edespino/gpdb,kaknikhil/gpdb,janebeckman/gpdb,adam8157/gpdb,kaknikhil/gpdb,kaknikhil/gpdb,edespino/gpdb,edespino/gpdb,rvs/gpdb,lisakowen/gpdb,kaknikhil/gpdb,Chibin/gpdb,jmcatamney/gpdb,edespino/gpdb,adam8157/gpdb,kaknikhil/gpdb,kaknikhil/gpdb,xinzweb/gpdb,ashwinstar/gpdb,yuanzhao/gpdb,edespino/gpdb,edespino/gpdb,lisakowen/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,cjcjameson/gpdb,jmcatamney/gpdb,kaknikhil/gpdb,yuanzhao/gpdb,50wu/gpdb,cjcjameson/gpdb,rvs/gpdb,rvs/gpdb,50wu/gpdb,kaknikhil/gpdb,rvs/gpdb,greenplum-db/gpdb,yuanzhao/gpdb,ashwinstar/gpdb,janebeckman/gpdb,adam8157/gpdb,Quikling/gpdb,jmcatamney/gpdb,yuanzhao/gpdb,cjcjameson/gpdb,jmcatamney/gpdb,yuanzhao/gpdb,greenplum-db/gpdb,Quikling/gpdb,greenplum-db/gpdb,yuanzhao/gpdb,janebeckman/gpdb,adam8157/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,lisakowen/gpdb,Chibin/gpdb,jmcatamney/gpdb,yuanzhao/gpdb,Quikling/gpdb,edespino/gpdb,rvs/gpdb,adam8157/gpdb,yuanzhao/gpdb,lisakowen/gpdb,ashwinstar/gpdb,xinzweb/gpdb,Chibin/gpdb,Quikling/gpdb,ashwinstar/gpdb,50wu/gpdb,lisakowen/gpdb,Quikling/gpdb,janebeckman/gpdb,cjcjameson/gpdb,janebeckman/gpdb,edespino/gpdb,Quikling/gpdb,kaknikhil/gpdb,Chibin/gpdb,janebeckman/gpdb,kaknikhil/gpdb,lisakowen/gpdb,50wu/gpdb,yuanzhao/gpdb,adam8157/gpdb,greenplum-db/gpdb,Quikling/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,Quikling/gpdb,50wu/gpdb,rvs/gpdb,lisakowen/gpdb,cjcjameson/gpdb,jmcatamney/gpdb,edespino/gpdb,yuanzhao/gpdb,xinzweb/gpdb,greenplum-db/gpdb,Chibin/gpdb,Chibin/gpdb,50wu/gpdb,cjcjameson/gpdb,janebeckman/gpdb