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/bin/node-template/runtime/src/lib.rs

@@ -157,6 +157,10 @@ impl system::Trait for Runtime {
 	type AvailableBlockRatio = AvailableBlockRatio;
 	/// Version of the runtime.
 	type Version = Version;
+	/// Converts a module to the index of the module in `construct_runtime!`.
+	///
+	/// This type is being generated by `construct_runtime!`.
+	type ModuleToIndex = ModuleToIndex;
 }
 
 impl aura::Trait for Runtime {

substrate/bin/node-template/runtime/src/template.rs

@@ -40,7 +40,7 @@ decl_module! {
 		// Just a dummy entry point.
 		// function that can be called by the external world as an extrinsics call
 		// takes a parameter of the type `AccountId`, stores it and emits an event
-		pub fn do_something(origin, something: u32) -> dispatch::Result {
+		pub fn do_something(origin, something: u32) -> dispatch::DispatchResult {
 			// TODO: You only need this if you want to check it was signed.
 			let who = ensure_signed(origin)?;
 
@@ -106,6 +106,7 @@ mod tests {
 		type MaximumBlockLength = MaximumBlockLength;
 		type AvailableBlockRatio = AvailableBlockRatio;
 		type Version = ();
+		type ModuleToIndex = ();
 	}
 	impl Trait for Test {
 		type Event = ();