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/im-online/src/tests.rs

@@ -111,7 +111,7 @@ fn heartbeat(
 	session_index: u32,
 	authority_index: u32,
 	id: UintAuthorityId,
-) -> dispatch::Result {
+) -> dispatch::DispatchResult {
 	#[allow(deprecated)]
 	use frame_support::unsigned::ValidateUnsigned;
 
@@ -127,7 +127,8 @@ fn heartbeat(
 	let signature = id.sign(&heartbeat.encode()).unwrap();
 
 	#[allow(deprecated)] // Allow ValidateUnsigned
-	ImOnline::pre_dispatch(&crate::Call::heartbeat(heartbeat.clone(), signature.clone()))?;
+	ImOnline::pre_dispatch(&crate::Call::heartbeat(heartbeat.clone(), signature.clone()))
+		.map_err(|e| <&'static str>::from(e))?;
 	ImOnline::heartbeat(
 		Origin::system(frame_system::RawOrigin::None),
 		heartbeat,