collation-generation: guide and tidying (#1753)

* Guide: Change for {Global,Local}ValidationData to ValidationData

Seems like that was missed during the transition from notions of global and local validation data to persistent and transient

* collation-generation: a small drive-by fix of line length

* collation-generation: extract an error module

this allows us to avoid piling Error and Result types from different contexts, specifically std vs. module-local

* collation-generation: extract LOG_TARGET from log statements

polkadot/node/collation-generation/src/error.rs

@@ -0,0 +1,32 @@
+// 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/>.
+
+
+#[derive(Debug, derive_more::From)]
+pub enum Error {
+	#[from]
+	Subsystem(polkadot_node_subsystem::SubsystemError),
+	#[from]
+	OneshotRecv(futures::channel::oneshot::Canceled),
+	#[from]
+	Runtime(polkadot_node_subsystem::errors::RuntimeApiError),
+	#[from]
+	Util(polkadot_node_subsystem_util::Error),
+	#[from]
+	Erasure(polkadot_erasure_coding::Error),
+}
+
+pub type Result<T> = std::result::Result<T, Error>;