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/reserved_keyword/on_initialize.rs

@@ -12,14 +12,14 @@ macro_rules! reserved {
 				pub mod system {
 					use frame_support::dispatch;
 
-					pub fn ensure_root<R>(_: R) -> dispatch::Result {
+					pub fn ensure_root<R>(_: R) -> dispatch::DispatchResult {
 						Ok(())
 					}
 				}
 
 				frame_support::decl_module! {
 					pub struct Module<T: Trait> for enum Call where origin: T::Origin {
-						fn $reserved(_origin) -> dispatch::Result { unreachable!() }
+						fn $reserved(_origin) -> dispatch::DispatchResult { unreachable!() }
 					}
 				}
 			}