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/ethereum/src/error.rs

@@ -0,0 +1,101 @@
+// 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 sp_runtime::RuntimeDebug;
+
+/// Header import error.
+#[derive(Clone, Copy, RuntimeDebug)]
+#[cfg_attr(feature = "std", derive(PartialEq))]
+pub enum Error {
+	/// The header is beyond last finalized and can not be imported.
+	AncientHeader = 0,
+	/// The header is already imported.
+	KnownHeader = 1,
+	/// Seal has an incorrect format.
+	InvalidSealArity = 2,
+	/// Block number isn't sensible.
+	RidiculousNumber = 3,
+	/// Block has too much gas used.
+	TooMuchGasUsed = 4,
+	/// Gas limit header field is invalid.
+	InvalidGasLimit = 5,
+	/// Extra data is of an invalid length.
+	ExtraDataOutOfBounds = 6,
+	/// Timestamp header overflowed.
+	TimestampOverflow = 7,
+	/// The parent header is missing from the blockchain.
+	MissingParentBlock = 8,
+	/// The header step is missing from the header.
+	MissingStep = 9,
+	/// The header signature is missing from the header.
+	MissingSignature = 10,
+	/// Empty steps are missing from the header.
+	MissingEmptySteps = 11,
+	/// The same author issued different votes at the same step.
+	DoubleVote = 12,
+	/// Validation proof insufficient.
+	InsufficientProof = 13,
+	/// Difficulty header field is invalid.
+	InvalidDifficulty = 14,
+	/// The received block is from an incorrect proposer.
+	NotValidator = 15,
+	/// Missing transaction receipts for the operation.
+	MissingTransactionsReceipts = 16,
+	/// Redundant transaction receipts are provided.
+	RedundantTransactionsReceipts = 17,
+	/// Provided transactions receipts are not matching the header.
+	TransactionsReceiptsMismatch = 18,
+	/// Can't accept unsigned header from the far future.
+	UnsignedTooFarInTheFuture = 19,
+	/// Trying to finalize sibling of finalized block.
+	TryingToFinalizeSibling = 20,
+	/// Header timestamp is ahead of on-chain timestamp
+	HeaderTimestampIsAhead = 21,
+}
+
+impl Error {
+	pub fn msg(&self) -> &'static str {
+		match *self {
+			Error::AncientHeader => "Header is beyound last finalized and can not be imported",
+			Error::KnownHeader => "Header is already imported",
+			Error::InvalidSealArity => "Header has an incorrect seal",
+			Error::RidiculousNumber => "Header has too large number",
+			Error::TooMuchGasUsed => "Header has too much gas used",
+			Error::InvalidGasLimit => "Header has invalid gas limit",
+			Error::ExtraDataOutOfBounds => "Header has too large extra data",
+			Error::TimestampOverflow => "Header has too large timestamp",
+			Error::MissingParentBlock => "Header has unknown parent hash",
+			Error::MissingStep => "Header is missing step seal",
+			Error::MissingSignature => "Header is missing signature seal",
+			Error::MissingEmptySteps => "Header is missing empty steps seal",
+			Error::DoubleVote => "Header has invalid step in seal",
+			Error::InsufficientProof => "Header has insufficient proof",
+			Error::InvalidDifficulty => "Header has invalid difficulty",
+			Error::NotValidator => "Header is sealed by unexpected validator",
+			Error::MissingTransactionsReceipts => "The import operation requires transactions receipts",
+			Error::RedundantTransactionsReceipts => "Redundant transactions receipts are provided",
+			Error::TransactionsReceiptsMismatch => "Invalid transactions receipts provided",
+			Error::UnsignedTooFarInTheFuture => "The unsigned header is too far in future",
+			Error::TryingToFinalizeSibling => "Trying to finalize sibling of finalized block",
+			Error::HeaderTimestampIsAhead => "Header timestamp is ahead of on-chain timestamp",
+		}
+	}
+
+	/// Return unique error code.
+	pub fn code(&self) -> u8 {
+		*self as u8
+	}
+}