Unify error enums in substrate and ethereum clients with thiserror (#1094)

* Unify error enums in substrate and ethereum clients with `thiserror`

Related to https://github.com/paritytech/parity-bridges-common/issues/857

* Add license pre-amble

* rustfmt

* Fix spelling

bridges/relays/utils/src/error.rs

@@ -0,0 +1,46 @@
+// Copyright 2019-2021 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 std::net::AddrParseError;
+use thiserror::Error;
+
+/// Result type used by relay utilities.
+pub type Result<T> = std::result::Result<T, Error>;
+
+/// Relay utilities errors.
+#[derive(Error, Debug)]
+pub enum Error {
+	/// Failed to request a float value from HTTP service.
+	#[error("Failed to fetch token price from remote server: {0}")]
+	FetchTokenPrice(#[source] anyhow::Error),
+	/// Failed to parse the response from HTTP service.
+	#[error("Failed to parse HTTP service response: {0:?}. Response: {1:?}")]
+	ParseHttp(serde_json::Error, String),
+	/// Failed to select response value from the JSON response.
+	#[error("Failed to select value from response: {0:?}. Response: {1:?}")]
+	SelectResponseValue(jsonpath_lib::JsonPathError, String),
+	/// Failed to parse float value from the selected value.
+	#[error(
+		"Failed to parse float value {0:?} from response. It is assumed to be positive and normal"
+	)]
+	ParseFloat(f64),
+	/// Couldn't found value in the JSON response.
+	#[error("Missing required value from response: {0:?}")]
+	MissingResponseValue(String),
+	/// Invalid host address was used for exposing Prometheus metrics.
+	#[error("Invalid host {0} is used to expose Prometheus metrics: {1}")]
+	ExposingMetricsInvalidHost(String, AddrParseError),
+	/// Prometheus error.
+	#[error("{0}")]
+	Prometheus(#[from] substrate_prometheus_endpoint::prometheus::Error),
+}