Add HTTP server error variant

src/util/error.rs

@@ -327,6 +327,9 @@ pub enum ErrorType {
     // and Fred is only available on the server
     #[error("Key-value store error: {0}")]
     KeyValStore(String),
+
+    #[error("HTTP server error: {0}")]
+    HttpServer(String),
 }
 
 impl From<ErrorType> for Error {