Fix broken ereport() call

Commit 2a3df5d0 introduced broken changes to the ereport() calls in the
external table command. Fix by including the proper format specifier.
Also concatenate the error message to a single row while at it to
improve readability.

Spotted by Heikki Linnakangas

src/backend/commands/exttablecmds.c

@@ -389,8 +389,7 @@ DefineExternalRelation(CreateExternalStmt *createExtStmt)
 				if (getgpsegmentCount() < list_length(exttypeDesc->location_list))
 					ereport(WARNING,
 							(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-							 errmsg("number of locations (%) exceeds the number "
-									"of segments (%d)",
+							 errmsg("number of locations (%d) exceeds the number of segments (%d)",
 									list_length(exttypeDesc->location_list),
 									getgpsegmentCount()),
 							 errhint("The table cannot be queried until cluster "