Chain API subsystem (#1498)

* chain-api subsystem skeleton

* chain-api subsystem: draft impl

* chain-api subsystem: mock testclient

* chain-api subsystem: impl HeaderBacked for TestClient

* chain-api subsystem: impl basic tests

* chain-api subsystem: tiny guide

* chain-api subsystem: rename ChainApiRequestMessage to ChainApiMessage

* chain-api subsystem: add the page to the ToC

* chain-api subsystem: proper error type

* chain-api subsystem: impl ancestors request

* chain-api subsystem: tests for ancestors request

* guide: fix ancestor return type

* runtime-api subsystem: remove unused dep

* fix fmt

* fix outdated comment

* chain-api subsystem: s/format/to_string

* lower-case subsystem names

* chain-api subsystem: resolve Finalized todo

* chain-api subsystem: remove TODO

* extract request errors into a module

* remove caching TODO

* fix imports

polkadot/node/subsystem/src/errors.rs

@@ -0,0 +1,57 @@
+// Copyright 2020 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/>.
+
+//! Error types for the subsystem requests.
+
+/// A description of an error causing the runtime API request to be unservable.
+#[derive(Debug, Clone)]
+pub struct RuntimeApiError(String);
+
+impl From<String> for RuntimeApiError {
+	fn from(s: String) -> Self {
+		RuntimeApiError(s)
+	}
+}
+
+impl core::fmt::Display for RuntimeApiError {
+	fn fmt(&self, f: &mut core::fmt::Formatter) -> Result<(), core::fmt::Error> {
+		write!(f, "{}", self.0)
+	}
+}
+
+/// A description of an error causing the chain API request to be unservable.
+#[derive(Debug, Clone)]
+pub struct ChainApiError {
+	msg: String,
+}
+
+impl From<&str> for ChainApiError {
+	fn from(s: &str) -> Self {
+		s.to_owned().into()
+	}
+}
+
+impl From<String> for ChainApiError {
+	fn from(msg: String) -> Self {
+		Self { msg }
+	}
+}
+
+impl core::fmt::Display for ChainApiError {
+	fn fmt(&self, f: &mut core::fmt::Formatter) -> Result<(), core::fmt::Error> {
+		write!(f, "{}", self.msg)
+	}
+}