CheckInherentError can now report when something would be valid (#1204)

* CheckInherentError can now report when something would be valid

* set timestamp inherent to next valid block time

* return max timestamp for valid-after when checking

substrate/core/sr-primitives/src/lib.rs

@@ -487,7 +487,10 @@ impl BasicInherentData {
 #[derive(Encode)]
 #[cfg_attr(feature = "std", derive(Decode))]
 pub enum CheckInherentError {
-	TimestampInFuture(u64),
+	/// The inherents are generally valid but a delay until the given timestamp
+	/// is required.
+	ValidAtTimestamp(u64),
+	/// Some other error has occurred.
 	Other(RuntimeString),
 }