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/example/src/lib.rs

@@ -255,7 +255,7 @@
 
 use sp_std::marker::PhantomData;
 use frame_support::{
-	dispatch::Result, decl_module, decl_storage, decl_event,
+	dispatch::DispatchResult, decl_module, decl_storage, decl_event,
 	weights::{SimpleDispatchInfo, DispatchInfo, DispatchClass, ClassifyDispatch, WeighData, Weight, PaysFee},
 };
 use frame_system::{self as system, ensure_signed, ensure_root};
@@ -460,7 +460,7 @@ decl_module! {
 		// transaction and the latter demonstrates the [`DispatchClass`] of the call. A higher
 		// weight means a larger transaction (less of which can be placed in a single block).
 		#[weight = SimpleDispatchInfo::FixedNormal(10_000)]
-		fn accumulate_dummy(origin, increase_by: T::Balance) -> Result {
+		fn accumulate_dummy(origin, increase_by: T::Balance) -> DispatchResult {
 			// This is a public call, so we ensure that the origin is some signed account.
 			let _sender = ensure_signed(origin)?;
 
@@ -543,7 +543,7 @@ decl_module! {
 impl<T: Trait> Module<T> {
 	// Add public immutables and private mutables.
 	#[allow(dead_code)]
-	fn accumulate_foo(origin: T::Origin, increase_by: T::Balance) -> Result {
+	fn accumulate_foo(origin: T::Origin, increase_by: T::Balance) -> DispatchResult {
 		let _sender = ensure_signed(origin)?;
 
 		let prev = <Foo<T>>::get();
@@ -685,6 +685,7 @@ mod tests {
 		type MaximumBlockLength = MaximumBlockLength;
 		type AvailableBlockRatio = AvailableBlockRatio;
 		type Version = ();
+		type ModuleToIndex = ();
 	}
 	parameter_types! {
 		pub const ExistentialDeposit: u64 = 0;