RPC & Client (#21)

* Initial version of http server.

* Refactor the structure.

* add unassigned ports info.

* Rename servers to match conventions.

* Add client crate and some proper RPC implementations.

* Style & docs.

substrate/client/src/error.rs

@@ -0,0 +1,49 @@
+// Copyright 2017 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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 Polkadot.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Polkadot client possible errors.
+
+use primitives::block;
+use state_machine;
+
+error_chain! {
+	errors {
+		/// Backend error.
+		Backend {
+			description("Unrecoverable backend error"),
+			display("Backend error"),
+		}
+
+		/// Unknown block.
+		UnknownBlock(h: block::HeaderHash) {
+			description("unknown block"),
+			display("UnknownBlock: {}", h),
+		}
+
+		/// Execution error.
+		Execution(e: Box<state_machine::Error>) {
+			description("execution error"),
+			display("Execution: {}", e),
+		}
+	}
+}
+
+// TODO [ToDr] Temporary, state_machine::Error should be a regular error not Box.
+impl From<Box<state_machine::Error>> for Error {
+	fn from(e: Box<state_machine::Error>) -> Self {
+		ErrorKind::Execution(e).into()
+	}
+}