Handle the gateway endpoint error

Related: #338

crates/gpapi/src/error.rs

@@ -0,0 +1,11 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum PortalError {
+  #[error("Portal prelogin error: {0}")]
+  PreloginError(String),
+  #[error("Portal config error: {0}")]
+  ConfigError(String),
+  #[error("Gateway error: {0}")]
+  GatewayError(String),
+}