lightclient: Add light client Error

Signed-off-by: Alexandru Vasile <alexandru.vasile@parity.io>

subxt/src/rpc/lightclient/mod.rs

@@ -0,0 +1,32 @@
+mod background;
+mod client;
+
+pub use client::LightClient;
+
+/// Light client error.
+#[derive(Debug, thiserror::Error)]
+pub enum LightClientError {
+    /// Error encountered while adding the chain to the light-client.
+    #[error("Failed to add the chain to the light client: {0}")]
+    AddChainError(String),
+    /// The background task is closed.
+    #[error("Failed to communicate with the background task.")]
+    BackgroundClosed,
+    /// Invalid RPC parameters cannot be serialized as JSON string.
+    #[error("RPC parameters cannot be serialized as JSON string.")]
+    InvalidParams,
+    /// Error originated while trying to submit a RPC request.
+    #[error("RPC request cannot be sent: {0}")]
+    Request(String),
+    /// The provided URL scheme is invalid.
+    ///
+    /// Supported versions: WS, WSS.
+    #[error("The provided URL scheme is invalid.")]
+    InvalidScheme,
+    /// The provided URL is invalid.
+    #[error("The provided URL scheme is invalid.")]
+    InvalidUrl,
+    /// Handshake error while connecting to a node.
+    #[error("WS handshake failed")]
+    Handshake,
+}