Unify error enums in substrate and ethereum clients with thiserror (#1094)

* Unify error enums in substrate and ethereum clients with `thiserror`

Related to https://github.com/paritytech/parity-bridges-common/issues/857

* Add license pre-amble

* rustfmt

* Fix spelling

bridges/relays/bin-substrate/src/chains/rococo.rs

@@ -14,6 +14,7 @@
 // You should have received a copy of the GNU General Public License
 // along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
 
+use anyhow::anyhow;
 use codec::Decode;
 use frame_support::weights::{DispatchClass, DispatchInfo, Pays, Weight};
 use relay_rococo_client::Rococo;
@@ -94,7 +95,7 @@ impl CliChain for Rococo {
 
 	fn encode_message(
 		_message: encode_message::MessagePayload,
-	) -> Result<Self::MessagePayload, String> {
-		Err("Sending messages from Rococo is not yet supported.".into())
+	) -> anyhow::Result<Self::MessagePayload> {
+		Err(anyhow!("Sending messages from Rococo is not yet supported."))
 	}
 }