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/modules/message-lane/rpc/src/error.rs

@@ -0,0 +1,59 @@
+// 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/>.
+
+//! Possible errors and results of message-lane RPC calls.
+
+/// Future Result type.
+pub type FutureResult<T> = jsonrpc_core::BoxFuture<T>;
+
+/// State RPC errors.
+#[derive(Debug, derive_more::Display, derive_more::From)]
+pub enum Error {
+	/// When unknown instance id is passed.
+	#[display(fmt = "Message lane instance is unknown")]
+	UnknownInstance,
+	/// Client error.
+	#[display(fmt = "Client error: {}", _0)]
+	Client(Box<dyn std::error::Error + Send>),
+}
+
+impl std::error::Error for Error {
+	fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+		match self {
+			Error::UnknownInstance => None,
+			Error::Client(ref err) => Some(&**err),
+		}
+	}
+}
+
+impl From<Error> for jsonrpc_core::Error {
+	fn from(e: Error) -> Self {
+		const UNKNOW_INSTANCE_CODE: i64 = 1;
+
+		match e {
+			Error::UnknownInstance => jsonrpc_core::Error {
+				code: jsonrpc_core::ErrorCode::ServerError(UNKNOW_INSTANCE_CODE),
+				message: "Unknown instance passed".into(),
+				data: None,
+			},
+			Error::Client(e) => jsonrpc_core::Error {
+				code: jsonrpc_core::ErrorCode::InternalError,
+				message: format!("Unknown error occured: {}", e),
+				data: Some(format!("{:?}", e).into()),
+			},
+		}
+	}
+}