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/elections-phragmen/src/lib.rs

@@ -83,9 +83,9 @@
 #![cfg_attr(not(feature = "std"), no_std)]
 
 use sp_std::prelude::*;
-use sp_runtime::{print, traits::{Zero, StaticLookup, Bounded, Convert}};
+use sp_runtime::{print, DispatchResult, traits::{Zero, StaticLookup, Bounded, Convert}};
 use frame_support::{
-	decl_storage, decl_event, ensure, decl_module, dispatch, weights::SimpleDispatchInfo,
+	decl_storage, decl_event, ensure, decl_module, weights::SimpleDispatchInfo,
 	traits::{
 		Currency, Get, LockableCurrency, LockIdentifier, ReservableCurrency, WithdrawReasons,
 		ChangeMembers, OnUnbalanced, WithdrawReason
@@ -373,7 +373,7 @@ decl_module! {
 				return Ok(());
 			}
 
-			return Err("origin is not a candidate, member or a runner up.");
+			Err("origin is not a candidate, member or a runner up.")?
 		}
 
 		/// Remove a particular member from the set. This is effective immediately and the bond of
@@ -390,7 +390,7 @@ decl_module! {
 		/// Writes: O(do_phragmen)
 		/// # </weight>
 		#[weight = SimpleDispatchInfo::FixedOperational(2_000_000)]
-		fn remove_member(origin, who: <T::Lookup as StaticLookup>::Source) -> dispatch::Result {
+		fn remove_member(origin, who: <T::Lookup as StaticLookup>::Source) -> DispatchResult {
 			ensure_root(origin)?;
 			let who = T::Lookup::lookup(who)?;
 
@@ -402,7 +402,7 @@ decl_module! {
 				if !had_replacement {
 					Self::do_phragmen();
 				}
-			})
+			}).map_err(Into::into)
 		}
 
 		/// What to do at the end of each block. Checks if an election needs to happen or not.
@@ -566,7 +566,7 @@ impl<T: Trait> Module<T> {
 	///
 	/// Runs phragmen election and cleans all the previous candidate state. The voter state is NOT
 	/// cleaned and voters must themselves submit a transaction to retract.
-	fn end_block(block_number: T::BlockNumber) -> dispatch::Result {
+	fn end_block(block_number: T::BlockNumber) -> DispatchResult {
 		if !Self::term_duration().is_zero() {
 			if (block_number % Self::term_duration()).is_zero() {
 				Self::do_phragmen();
@@ -768,6 +768,7 @@ mod tests {
 		type MaximumBlockLength = MaximumBlockLength;
 		type AvailableBlockRatio = AvailableBlockRatio;
 		type Version = ();
+		type ModuleToIndex = ();
 	}
 
 	parameter_types! {