Adding Bridges code as git subtree. (#2515)

* Add instructions.

* Squashed 'bridges/' content from commit 345e84a21

git-subtree-dir: bridges
git-subtree-split: 345e84a2146b56628e9888c9f5e129cb40e868a9

* Remove bridges workspace file to avoid confusing Cargo.

* Add some bridges primitives to Polkadot workspace.

* Improve docs.

polkadot/bridges/relays/ethereum/src/rpc_errors.rs

@@ -0,0 +1,82 @@
+// Copyright 2019-2020 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity Bridges Common is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// 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 relay_ethereum_client::Error as EthereumNodeError;
+use relay_substrate_client::Error as SubstrateNodeError;
+use relay_utils::MaybeConnectionError;
+
+/// Contains common errors that can occur when
+/// interacting with a Substrate or Ethereum node
+/// through RPC.
+#[derive(Debug)]
+pub enum RpcError {
+	/// The arguments to the RPC method failed to serialize.
+	Serialization(serde_json::Error),
+	/// An error occured when interacting with an Ethereum node.
+	Ethereum(EthereumNodeError),
+	/// An error occured when interacting with a Substrate node.
+	Substrate(SubstrateNodeError),
+}
+
+impl From<RpcError> for String {
+	fn from(err: RpcError) -> Self {
+		match err {
+			RpcError::Serialization(e) => e.to_string(),
+			RpcError::Ethereum(e) => e.to_string(),
+			RpcError::Substrate(e) => e.to_string(),
+		}
+	}
+}
+
+impl From<serde_json::Error> for RpcError {
+	fn from(err: serde_json::Error) -> Self {
+		Self::Serialization(err)
+	}
+}
+
+impl From<EthereumNodeError> for RpcError {
+	fn from(err: EthereumNodeError) -> Self {
+		Self::Ethereum(err)
+	}
+}
+
+impl From<SubstrateNodeError> for RpcError {
+	fn from(err: SubstrateNodeError) -> Self {
+		Self::Substrate(err)
+	}
+}
+
+impl From<ethabi::Error> for RpcError {
+	fn from(err: ethabi::Error) -> Self {
+		Self::Ethereum(EthereumNodeError::ResponseParseFailed(format!("{}", err)))
+	}
+}
+
+impl MaybeConnectionError for RpcError {
+	fn is_connection_error(&self) -> bool {
+		match self {
+			RpcError::Ethereum(ref error) => error.is_connection_error(),
+			RpcError::Substrate(ref error) => error.is_connection_error(),
+			_ => false,
+		}
+	}
+}
+
+impl From<codec::Error> for RpcError {
+	fn from(err: codec::Error) -> Self {
+		Self::Substrate(SubstrateNodeError::ResponseParseFailed(err))
+	}
+}