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/scored-pool/src/lib.rs

@@ -61,7 +61,7 @@
 //!
 //! decl_module! {
 //! 	pub struct Module<T: Trait> for enum Call where origin: T::Origin {
-//! 		pub fn candidate(origin) -> dispatch::Result {
+//! 		pub fn candidate(origin) -> dispatch::DispatchResult {
 //! 			let who = ensure_signed(origin)?;
 //!
 //! 			let _ = <scored_pool::Module<T>>::submit_candidacy(
@@ -261,7 +261,7 @@ decl_module! {
 			// `None` are always sorted to the end.
 			if let Err(e) = <Pool<T, I>>::append(&[(who.clone(), None)]) {
 				T::Currency::unreserve(&who, deposit);
-				return Err(e);
+				Err(e)?
 			}
 
 			<CandidateExists<T, I>>::insert(&who, true);

substrate/frame/scored-pool/src/mock.rs

@@ -70,6 +70,7 @@ impl frame_system::Trait for Test {
 	type MaximumBlockLength = MaximumBlockLength;
 	type AvailableBlockRatio = AvailableBlockRatio;
 	type Version = ();
+	type ModuleToIndex = ();
 }
 
 impl pallet_balances::Trait for Test {