Fix message style

Mark one message not for translation, and prefer "cannot" over "may
not", per commentary from Robert Haas.

Discussion: https://postgr.es/m/20190430145813.GA29872@alvherre.pgsql

src/backend/access/heap/heapam.c

@@ -1301,7 +1301,7 @@ heap_getnext(TableScanDesc sscan, ScanDirection direction)
 	if (unlikely(sscan->rs_rd->rd_tableam != GetHeapamTableAmRoutine()))
 		ereport(ERROR,
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-				 errmsg("only heap AM is supported")));
+				 errmsg_internal("only heap AM is supported")));
 
 	/* Note: no locking manipulations needed */
 

src/backend/access/table/tableamapi.c

@@ -106,7 +106,8 @@ check_default_table_access_method(char **newval, void **extra, GucSource source)
 {
 	if (**newval == '\0')
 	{
-		GUC_check_errdetail("default_table_access_method may not be empty.");
+		GUC_check_errdetail("%s cannot be empty.",
+							"default_table_access_method");
 		return false;
 	}
 

src/test/regress/expected/create_am.out

@@ -110,7 +110,7 @@ NOTICE:  drop cascades to index grect2ind2
 -- prevent empty values
 SET default_table_access_method = '';
 ERROR:  invalid value for parameter "default_table_access_method": ""
-DETAIL:  default_table_access_method may not be empty.
+DETAIL:  default_table_access_method cannot be empty.
 -- prevent nonexistant values
 SET default_table_access_method = 'I do not exist AM';
 ERROR:  invalid value for parameter "default_table_access_method": "I do not exist AM"