Set error code for "incompatible loci in target inheritance set" error.

There is a test case that reaches this, in the 'file_fdw' test. With
ERRCODE_INTERNAL_ERROR, the error message includes the source file location
(planner.c:1513) in the error message. That's problematic, because the line
number changes whenever we touch planner.c.

Since this error is in fact reachable, mark it as FEATURE_NOT_SUPPORTED.

contrib/file_fdw/output/file_fdw.source

@@ -276,9 +276,9 @@ SELECT tableoid::regclass, * FROM ONLY agg;
 
 -- updates aren't supported
 UPDATE agg SET a = 1;
-ERROR:  incompatible loci in target inheritance set (planner.c:1513)
+ERROR:  incompatible loci in target inheritance set
 DELETE FROM agg WHERE a = 100;
-ERROR:  incompatible loci in target inheritance set (planner.c:1513)
+ERROR:  incompatible loci in target inheritance set
 -- but this should be allowed
 SELECT tableoid::regclass, * FROM agg FOR UPDATE;
  tableoid |  a  |    b    

contrib/file_fdw/output/file_fdw_optimizer.source

@@ -276,9 +276,9 @@ SELECT tableoid::regclass, * FROM ONLY agg;
 
 -- updates aren't supported
 UPDATE agg SET a = 1;
-ERROR:  incompatible loci in target inheritance set (planner.c:1513)
+ERROR:  incompatible loci in target inheritance set
 DELETE FROM agg WHERE a = 100;
-ERROR:  incompatible loci in target inheritance set (planner.c:1513)
+ERROR:  incompatible loci in target inheritance set
 -- but this should be allowed
 SELECT tableoid::regclass, * FROM agg FOR UPDATE;
  tableoid |  a  |    b    

src/backend/optimizer/plan/planner.c

@@ -1508,9 +1508,14 @@ inheritance_planner(PlannerInfo *root)
 			}
 			if (!locus_ok)
 			{
-				ereport(ERROR, (
-								errcode(ERRCODE_INTERNAL_ERROR),
-					 errmsg("incompatible loci in target inheritance set")));
+				/*
+				 * This is reachable, if you have normal distributed/replicated tables,
+				 * and foreign tables that can only be executed in the QD, in the same
+				 * inheritance tree.
+				 */
+				ereport(ERROR,
+						(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+						 errmsg("incompatible loci in target inheritance set")));
 			}
 		}