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/mod.rs

@@ -161,6 +161,7 @@ mod tests {
 	use wabt;
 	use hex_literal::hex;
 	use assert_matches::assert_matches;
+	use sp_runtime::DispatchError;
 
 	#[derive(Debug, PartialEq, Eq)]
 	struct DispatchEntry(Call);
@@ -1429,7 +1430,7 @@ mod tests {
 				MockExt::default(),
 				&mut gas_meter
 			),
-			Err(ExecError { reason: "during execution", buffer: _ })
+			Err(ExecError { reason: DispatchError::Other("during execution"), buffer: _ })
 		);
 	}
 
@@ -1471,7 +1472,7 @@ mod tests {
 				MockExt::default(),
 				&mut gas_meter
 			),
-			Err(ExecError { reason: "during execution", buffer: _ })
+			Err(ExecError { reason: DispatchError::Other("during execution"), buffer: _ })
 		);
 	}