Reject nonzero day fields in AT TIME ZONE INTERVAL functions.

It's not sensible for an interval that's used as a time zone value to be
larger than a day.  When we changed the interval type to contain a separate
day field, check_timezone() was adjusted to reject nonzero day values, but
timetz_izone(), timestamp_izone(), and timestamptz_izone() evidently were
overlooked.

While at it, make the error messages for these three cases consistent.

src/backend/utils/adt/date.c

@@ -2696,10 +2696,10 @@ timetz_izone(PG_FUNCTION_ARGS)
 	TimeTzADT  *result;
 	int			tz;
 
-	if (zone->month != 0)
+	if (zone->month != 0 || zone->day != 0)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-				 errmsg("\"interval\" time zone \"%s\" not valid",
+				 errmsg("interval time zone \"%s\" must not include months or days",
 						DatumGetCString(DirectFunctionCall1(interval_out,
 												  PointerGetDatum(zone))))));
 

src/backend/utils/adt/timestamp.c

@@ -4604,10 +4604,10 @@ timestamp_izone(PG_FUNCTION_ARGS)
 	if (TIMESTAMP_NOT_FINITE(timestamp))
 		PG_RETURN_TIMESTAMPTZ(timestamp);
 
-	if (zone->month != 0)
+	if (zone->month != 0 || zone->day != 0)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-				 errmsg("interval time zone \"%s\" must not specify month",
+				 errmsg("interval time zone \"%s\" must not include months or days",
 						DatumGetCString(DirectFunctionCall1(interval_out,
 												  PointerGetDatum(zone))))));
 
@@ -4777,10 +4777,10 @@ timestamptz_izone(PG_FUNCTION_ARGS)
 	if (TIMESTAMP_NOT_FINITE(timestamp))
 		PG_RETURN_TIMESTAMP(timestamp);
 
-	if (zone->month != 0)
+	if (zone->month != 0 || zone->day != 0)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-				 errmsg("interval time zone \"%s\" must not specify month",
+				 errmsg("interval time zone \"%s\" must not include months or days",
 						DatumGetCString(DirectFunctionCall1(interval_out,
 												  PointerGetDatum(zone))))));