Change a couple of "can't happen" error messages to be a shade more verbose when they do happen.  The "left link changed unexpectedly" one in particular has been seen more than once in the field.
Change a couple of "can't happen" error messages to be a shade more
verbose when they do happen.  The "left link changed unexpectedly"
one in particular has been seen more than once in the field.

chrishajas/gpdb,Chibin/gpdb,tangp3/gpdb,ashwinstar/gpdb,randomtask1155/gpdb,techdragon/Postgres-XL,ahachete/gpdb,edespino/gpdb,0x0FFF/gpdb,xuegang/gpdb,tpostgres-projects/tPostgres,xinzweb/gpdb,greenplum-db/gpdb,foyzur/gpdb,royc1/gpdb,rvs/gpdb,techdragon/Postgres-XL,Chibin/gpdb,lisakowen/gpdb,ashwinstar/gpdb,lisakowen/gpdb,atris/gpdb,jmcatamney/gpdb,kaknikhil/gpdb,0x0FFF/gpdb,yuanzhao/gpdb,rvs/gpdb,lisakowen/gpdb,oberstet/postgres-xl,Quikling/gpdb,Chibin/gpdb,tangp3/gpdb,xinzweb/gpdb,edespino/gpdb,CraigHarris/gpdb,Chibin/gpdb,CraigHarris/gpdb,ahachete/gpdb,yuanzhao/gpdb,randomtask1155/gpdb,oberstet/postgres-xl,CraigHarris/gpdb,adam8157/gpdb,royc1/gpdb,adam8157/gpdb,Quikling/gpdb,0x0FFF/gpdb,atris/gpdb,pavanvd/postgres-xl,postmind-net/postgres-xl,oberstet/postgres-xl,snaga/postgres-xl,atris/gpdb,tangp3/gpdb,techdragon/Postgres-XL,randomtask1155/gpdb,snaga/postgres-xl,Quikling/gpdb,janebeckman/gpdb,cjcjameson/gpdb,xuegang/gpdb,arcivanov/postgres-xl,cjcjameson/gpdb,royc1/gpdb,greenplum-db/gpdb,cjcjameson/gpdb,ashwinstar/gpdb,xuegang/gpdb,royc1/gpdb,lintzc/gpdb,adam8157/gpdb,adam8157/gpdb,Chibin/gpdb,janebeckman/gpdb,ahachete/gpdb,foyzur/gpdb,tpostgres-projects/tPostgres,adam8157/gpdb,xinzweb/gpdb,zeroae/postgres-xl,Chibin/gpdb,postmind-net/postgres-xl,yazun/postgres-xl,arcivanov/postgres-xl,rubikloud/gpdb,xuegang/gpdb,chrishajas/gpdb,techdragon/Postgres-XL,xinzweb/gpdb,ahachete/gpdb,ahachete/gpdb,foyzur/gpdb,lintzc/gpdb,edespino/gpdb,0x0FFF/gpdb,lintzc/gpdb,kmjungersen/PostgresXL,lpetrov-pivotal/gpdb,rvs/gpdb,lisakowen/gpdb,chrishajas/gpdb,ovr/postgres-xl,lpetrov-pivotal/gpdb,Postgres-XL/Postgres-XL,Quikling/gpdb,0x0FFF/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,adam8157/gpdb,CraigHarris/gpdb,kaknikhil/gpdb,greenplum-db/gpdb,kaknikhil/gpdb,zeroae/postgres-xl,atris/gpdb,lintzc/gpdb,zaksoup/gpdb,randomtask1155/gpdb,0x0FFF/gpdb,edespino/gpdb,janebeckman/gpdb,adam8157/gpdb,ahachete/gpdb,50wu/gpdb,yuanzhao/gpdb,lintzc/gpdb,rvs/gpdb,techdragon/Postgres-XL,kmjungersen/PostgresXL,zaksoup/gpdb,tangp3/gpdb,royc1/gpdb,chrishajas/gpdb,lpetrov-pivotal/gpdb,Quikling/gpdb,rvs/gpdb,zaksoup/gpdb,royc1/gpdb,CraigHarris/gpdb,yuanzhao/gpdb,randomtask1155/gpdb,ovr/postgres-xl,arcivanov/postgres-xl,yuanzhao/gpdb,cjcjameson/gpdb,foyzur/gpdb,postmind-net/postgres-xl,tangp3/gpdb,lpetrov-pivotal/gpdb,jmcatamney/gpdb,janebeckman/gpdb,lintzc/gpdb,ahachete/gpdb,lisakowen/gpdb,rubikloud/gpdb,atris/gpdb,edespino/gpdb,janebeckman/gpdb,janebeckman/gpdb,lisakowen/gpdb,tpostgres-projects/tPostgres,jmcatamney/gpdb,greenplum-db/gpdb,yazun/postgres-xl,kaknikhil/gpdb,yazun/postgres-xl,Postgres-XL/Postgres-XL,ashwinstar/gpdb,Quikling/gpdb,yazun/postgres-xl,lisakowen/gpdb,kmjungersen/PostgresXL,arcivanov/postgres-xl,zaksoup/gpdb,jmcatamney/gpdb,snaga/postgres-xl,jmcatamney/gpdb,zaksoup/gpdb,arcivanov/postgres-xl,oberstet/postgres-xl,yuanzhao/gpdb,chrishajas/gpdb,postmind-net/postgres-xl,Chibin/gpdb,snaga/postgres-xl,ovr/postgres-xl,ovr/postgres-xl,pavanvd/postgres-xl,zaksoup/gpdb,CraigHarris/gpdb,cjcjameson/gpdb,tangp3/gpdb,yuanzhao/gpdb,50wu/gpdb,0x0FFF/gpdb,rubikloud/gpdb,randomtask1155/gpdb,edespino/gpdb,jmcatamney/gpdb,rubikloud/gpdb,50wu/gpdb,edespino/gpdb,yuanzhao/gpdb,kaknikhil/gpdb,rvs/gpdb,cjcjameson/gpdb,cjcjameson/gpdb,50wu/gpdb,kaknikhil/gpdb,arcivanov/postgres-xl,pavanvd/postgres-xl,xinzweb/gpdb,atris/gpdb,pavanvd/postgres-xl,kaknikhil/gpdb,randomtask1155/gpdb,ahachete/gpdb,xuegang/gpdb,chrishajas/gpdb,yazun/postgres-xl,lpetrov-pivotal/gpdb,zeroae/postgres-xl,foyzur/gpdb,tangp3/gpdb,xinzweb/gpdb,atris/gpdb,zeroae/postgres-xl,chrishajas/gpdb,lintzc/gpdb,yuanzhao/gpdb,xuegang/gpdb,50wu/gpdb,greenplum-db/gpdb,xuegang/gpdb,royc1/gpdb,0x0FFF/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,lintzc/gpdb,rvs/gpdb,rubikloud/gpdb,greenplum-db/gpdb,Quikling/gpdb,tangp3/gpdb,50wu/gpdb,cjcjameson/gpdb,rubikloud/gpdb,lpetrov-pivotal/gpdb,oberstet/postgres-xl,rvs/gpdb,royc1/gpdb,kaknikhil/gpdb,zaksoup/gpdb,atris/gpdb,50wu/gpdb,pavanvd/postgres-xl,CraigHarris/gpdb,tpostgres-projects/tPostgres,lpetrov-pivotal/gpdb,Quikling/gpdb,chrishajas/gpdb,rubikloud/gpdb,Quikling/gpdb,postmind-net/postgres-xl,jmcatamney/gpdb,snaga/postgres-xl,kmjungersen/PostgresXL,ashwinstar/gpdb,xinzweb/gpdb,Postgres-XL/Postgres-XL,jmcatamney/gpdb,yuanzhao/gpdb,janebeckman/gpdb,50wu/gpdb,ovr/postgres-xl,Chibin/gpdb,randomtask1155/gpdb,foyzur/gpdb,lpetrov-pivotal/gpdb,Chibin/gpdb,rvs/gpdb,janebeckman/gpdb,edespino/gpdb,foyzur/gpdb,ashwinstar/gpdb,Postgres-XL/Postgres-XL,zeroae/postgres-xl,xinzweb/gpdb,Chibin/gpdb,lisakowen/gpdb,tpostgres-projects/tPostgres,CraigHarris/gpdb,zaksoup/gpdb,CraigHarris/gpdb,kaknikhil/gpdb,cjcjameson/gpdb,edespino/gpdb,rubikloud/gpdb,Postgres-XL/Postgres-XL,Quikling/gpdb,janebeckman/gpdb,kaknikhil/gpdb,foyzur/gpdb,cjcjameson/gpdb,xuegang/gpdb,janebeckman/gpdb,rvs/gpdb,xuegang/gpdb,adam8157/gpdb,lintzc/gpdb,edespino/gpdb,kmjungersen/PostgresXL