Use a consistent error message style for user mappings.

User mappings are essentially anonymous, so messages referring to "user
mapping foo on server bar" are wrong, and inconsistent with other error
messages referring to user mappings. To be consistent with existing use,
use "user mapping for foo on server bar" instead.

I dropped the noise word "user" from the original suggestion to be
consistent with other uses.

Discussion: http://postgr.es/m/56c6f8ab-b2d6-f1fa-deb0-1d18cf67f7b9@2ndQuadrant.com

src/backend/commands/foreigncmds.c

@@ -1155,7 +1155,7 @@ CreateUserMapping(CreateUserMappingStmt *stmt)
 	if (OidIsValid(umId))
 		ereport(ERROR,
 				(errcode(ERRCODE_DUPLICATE_OBJECT),
-				 errmsg("user mapping \"%s\" already exists for server %s",
+				 errmsg("user mapping for \"%s\" already exists for server %s",
 						MappingUserName(useId),
 						stmt->servername)));
 
@@ -1247,7 +1247,7 @@ AlterUserMapping(AlterUserMappingStmt *stmt)
 	if (!OidIsValid(umId))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_OBJECT),
-				 errmsg("user mapping \"%s\" does not exist for the server",
+				 errmsg("user mapping for \"%s\" does not exist for the server",
 						MappingUserName(useId))));
 
 	user_mapping_ddl_aclcheck(useId, srv->serverid, stmt->servername);
@@ -1362,12 +1362,12 @@ RemoveUserMapping(DropUserMappingStmt *stmt)
 		if (!stmt->missing_ok)
 			ereport(ERROR,
 					(errcode(ERRCODE_UNDEFINED_OBJECT),
-				  errmsg("user mapping \"%s\" does not exist for the server",
+				  errmsg("user mapping for \"%s\" does not exist for the server",
 						 MappingUserName(useId))));
 
 		/* IF EXISTS specified, just note it */
 		ereport(NOTICE,
-		(errmsg("user mapping \"%s\" does not exist for the server, skipping",
+		(errmsg("user mapping for \"%s\" does not exist for the server, skipping",
 				MappingUserName(useId))));
 		return InvalidOid;
 	}

src/test/regress/expected/foreign_data.out

@@ -556,7 +556,7 @@ CREATE USER MAPPING FOR current_user SERVER s1;             -- ERROR
 ERROR:  server "s1" does not exist
 CREATE USER MAPPING FOR current_user SERVER s4;
 CREATE USER MAPPING FOR user SERVER s4;                     -- ERROR duplicate
-ERROR:  user mapping "regress_foreign_data_user" already exists for server s4
+ERROR:  user mapping for "regress_foreign_data_user" already exists for server s4
 CREATE USER MAPPING FOR public SERVER s4 OPTIONS ("this mapping" 'is public');
 CREATE USER MAPPING FOR user SERVER s8 OPTIONS (username 'test', password 'secret');    -- ERROR
 ERROR:  invalid option "username"
@@ -596,7 +596,7 @@ ERROR:  role "regress_test_missing_role" does not exist
 ALTER USER MAPPING FOR user SERVER ss4 OPTIONS (gotcha 'true'); -- ERROR
 ERROR:  server "ss4" does not exist
 ALTER USER MAPPING FOR public SERVER s5 OPTIONS (gotcha 'true');            -- ERROR
-ERROR:  user mapping "public" does not exist for the server
+ERROR:  user mapping for "public" does not exist for the server
 ALTER USER MAPPING FOR current_user SERVER s8 OPTIONS (username 'test');    -- ERROR
 ERROR:  invalid option "username"
 HINT:  Valid options in this context are: user, password
@@ -626,13 +626,13 @@ ERROR:  role "regress_test_missing_role" does not exist
 DROP USER MAPPING FOR user SERVER ss4;
 ERROR:  server "ss4" does not exist
 DROP USER MAPPING FOR public SERVER s7;                     -- ERROR
-ERROR:  user mapping "public" does not exist for the server
+ERROR:  user mapping for "public" does not exist for the server
 DROP USER MAPPING IF EXISTS FOR regress_test_missing_role SERVER s4;
 NOTICE:  role "regress_test_missing_role" does not exist, skipping
 DROP USER MAPPING IF EXISTS FOR user SERVER ss4;
 NOTICE:  server does not exist, skipping
 DROP USER MAPPING IF EXISTS FOR public SERVER s7;
-NOTICE:  user mapping "public" does not exist for the server, skipping
+NOTICE:  user mapping for "public" does not exist for the server, skipping
 CREATE USER MAPPING FOR public SERVER s8;
 SET ROLE regress_test_role;
 DROP USER MAPPING FOR public SERVER s8;                     -- ERROR