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/balances/src/tests.rs

@@ -18,7 +18,7 @@
 
 use super::*;
 use mock::{Balances, ExtBuilder, Runtime, System, info_from_weight, CALL};
-use sp_runtime::traits::SignedExtension;
+use sp_runtime::traits::{SignedExtension, BadOrigin};
 use frame_support::{
 	assert_noop, assert_ok, assert_err,
 	traits::{LockableCurrency, LockIdentifier, WithdrawReason, WithdrawReasons,
@@ -347,7 +347,7 @@ fn force_transfer_works() {
 		let _ = Balances::deposit_creating(&1, 111);
 		assert_noop!(
 			Balances::force_transfer(Some(2).into(), 1, 2, 69),
-			"RequireRootOrigin",
+			BadOrigin,
 		);
 		assert_ok!(Balances::force_transfer(RawOrigin::Root.into(), 1, 2, 69));
 		assert_eq!(Balances::total_balance(&1), 42);