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/procedural/tools/src/lib.rs

@@ -47,7 +47,7 @@ pub fn generate_crate_access(unique_id: &str, def_crate: &str) -> TokenStream {
 
 /// Generates the hidden includes that are required to make the macro independent from its scope.
 pub fn generate_hidden_includes(unique_id: &str, def_crate: &str) -> TokenStream {
-	if ::std::env::var("CARGO_PKG_NAME").unwrap() == def_crate {
+	if std::env::var("CARGO_PKG_NAME").unwrap() == def_crate {
 		TokenStream::new()
 	} else {
 		let mod_name = generate_hidden_includes_mod_name(unique_id);