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/gas.rs

@@ -21,6 +21,7 @@ use sp_runtime::traits::{
 };
 use frame_support::{
 	traits::{Currency, ExistenceRequirement, Imbalance, OnUnbalanced, WithdrawReason}, StorageValue,
+	dispatch::DispatchError,
 };
 
 #[cfg(test)]
@@ -201,7 +202,7 @@ impl<T: Trait> GasMeter<T> {
 pub fn buy_gas<T: Trait>(
 	transactor: &T::AccountId,
 	gas_limit: Gas,
-) -> Result<(GasMeter<T>, NegativeImbalanceOf<T>), &'static str> {
+) -> Result<(GasMeter<T>, NegativeImbalanceOf<T>), DispatchError> {
 	// Buy the specified amount of gas.
 	let gas_price = <Module<T>>::gas_price();
 	let cost = if gas_price.is_zero() {