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/src/lib.rs

@@ -124,7 +124,7 @@ pub use frame_support_procedural::{decl_storage, construct_runtime};
 #[macro_export]
 macro_rules! fail {
 	( $y:expr ) => {{
-		return Err($y);
+		return Err($y.into());
 	}}
 }
 
@@ -168,7 +168,7 @@ macro_rules! assert_noop {
 #[cfg(feature = "std")]
 macro_rules! assert_err {
 	( $x:expr , $y:expr $(,)? ) => {
-		assert_eq!($x, Err($y));
+		assert_eq!($x, Err($y.into()));
 	}
 }