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/support/test/tests/system.rs

@@ -8,6 +8,7 @@ pub trait Trait: 'static + Eq + Clone {
 	type Hash;
 	type AccountId: Encode + EncodeLike + Decode;
 	type Event: From<Event>;
+	type ModuleToIndex: frame_support::traits::ModuleToIndex;
 }
 
 frame_support::decl_module! {
@@ -28,7 +29,7 @@ frame_support::decl_event!(
 );
 
 frame_support::decl_error! {
-	pub enum Error {
+	pub enum Error for Module<T: Trait> {
 		/// Test error documentation
 		TestError,
 		/// Error documentation