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/contracts/src/wasm/runtime.rs

@@ -98,7 +98,7 @@ pub(crate) fn to_execution_result<E: Ext>(
 		// validated by the code preparation process. However, because panics are really
 		// undesirable in the runtime code, we treat this as a trap for now. Eventually, we might
 		// want to revisit this.
-		Ok(_) => Err(ExecError { reason: "return type error", buffer: runtime.scratch_buf }),
+		Ok(_) => Err(ExecError { reason: "return type error".into(), buffer: runtime.scratch_buf }),
 		// `Error::Module` is returned only if instantiation or linking failed (i.e.
 		// wasm binary tried to import a function that is not provided by the host).
 		// This shouldn't happen because validation process ought to reject such binaries.
@@ -106,10 +106,10 @@ pub(crate) fn to_execution_result<E: Ext>(
 		// Because panics are really undesirable in the runtime code, we treat this as
 		// a trap for now. Eventually, we might want to revisit this.
 		Err(sp_sandbox::Error::Module) =>
-			Err(ExecError { reason: "validation error", buffer: runtime.scratch_buf }),
+			Err(ExecError { reason: "validation error".into(), buffer: runtime.scratch_buf }),
 		// Any other kind of a trap should result in a failure.
 		Err(sp_sandbox::Error::Execution) | Err(sp_sandbox::Error::OutOfBounds) =>
-			Err(ExecError { reason: "during execution", buffer: runtime.scratch_buf }),
+			Err(ExecError { reason: "during execution".into(), buffer: runtime.scratch_buf }),
 	}
 }