Make decl_error! errors usable (#4449)

* Make `decl_error!` errors usable

This pr implements support for returning errors of different pallets in
a pallet. These errors need to be declared with `decl_error!`.

The pr changes the following:

- Each dispatchable function now returns a `DispatchResult` which is an
alias for `Result<(), DispatchError>`.
- `DispatchError` is an enum that has 4 variants:
  - `Other`: For storing string error messages
  - `CannotLookup`: Variant that is returned when something returns a
  `sp_runtime::LookupError`
  - `BadOrigin`: Variant that is returned for any kind of bad origin
  - `Module`: The error of a specific module. Contains the `index`,
  `error` and the `message`. The index is the index of the module in
  `construct_runtime!`. `error` is the index of the error in the error
  enum declared by `decl_error!`. `message` is the message to the error
  variant (this will not be encoded).
- `construct_runtime!` now creates a new struct `ModuleToIndex`. This
struct implements the trait `ModuleToIndex`.
- `frame_system::Trait` has a new associated type: `ModuleToIndex` that
expects the `ModuleToIndex` generated by `construct_runtime!`.
- All error strings returned in any module are being converted now to `DispatchError`.
- `BadOrigin` is the default error returned by any type that implements `EnsureOrigin`.

* Fix frame system benchmarks

substrate/frame/session/src/lib.rs

@@ -483,14 +483,14 @@ decl_module! {
 		/// - One extra DB entry.
 		/// # </weight>
 		#[weight = SimpleDispatchInfo::FixedNormal(150_000)]
-		fn set_keys(origin, keys: T::Keys, proof: Vec<u8>) -> dispatch::Result {
+		fn set_keys(origin, keys: T::Keys, proof: Vec<u8>) -> dispatch::DispatchResult {
 			let who = ensure_signed(origin)?;
 
 			ensure!(keys.ownership_proof_is_valid(&proof), "invalid ownership proof");
 
 			let who = match T::ValidatorIdOf::convert(who) {
 				Some(val_id) => val_id,
-				None => return Err("no associated validator ID for account."),
+				None => Err("no associated validator ID for account.")?,
 			};
 
 			Self::do_set_keys(&who, keys)?;
@@ -631,7 +631,7 @@ impl<T: Trait> Module<T> {
 
 	// perform the set_key operation, checking for duplicates.
 	// does not set `Changed`.
-	fn do_set_keys(who: &T::ValidatorId, keys: T::Keys) -> dispatch::Result {
+	fn do_set_keys(who: &T::ValidatorId, keys: T::Keys) -> dispatch::DispatchResult {
 		let old_keys = Self::load_keys(&who);
 
 		for id in T::Keys::key_ids() {

substrate/frame/session/src/mock.rs

@@ -174,6 +174,7 @@ impl frame_system::Trait for Test {
 	type AvailableBlockRatio = AvailableBlockRatio;
 	type MaximumBlockLength = MaximumBlockLength;
 	type Version = ();
+	type ModuleToIndex = ();
 }
 
 impl pallet_timestamp::Trait for Test {