Improve error reporting when header is too long.

Refs #1471.

Author: aaugustin

src/websockets/legacy/server.py

@@ -199,10 +199,14 @@ async def handler(self) -> None:
                 elif isinstance(exc, InvalidHandshake):
                     if self.debug:
                         self.logger.debug("! invalid handshake", exc_info=True)
+                    exc_str = f"{exc}"
+                    while exc.__cause__ is not None:
+                        exc = exc.__cause__
+                        exc_str += f"; {exc}"
                     status, headers, body = (
                         http.HTTPStatus.BAD_REQUEST,
                         Headers(),
-                        f"Failed to open a WebSocket connection: {exc}.\n".encode(),
+                        f"Failed to open a WebSocket connection: {exc_str}.\n".encode(),
                     )
                 else:
                     self.logger.error("opening handshake failed", exc_info=True)

src/websockets/server.py

@@ -163,9 +163,13 @@ def accept(self, request: Request) -> Response:
             self.handshake_exc = exc
             if self.debug:
                 self.logger.debug("! invalid handshake", exc_info=True)
+            exc_str = f"{exc}"
+            while exc.__cause__ is not None:
+                exc = exc.__cause__
+                exc_str += f"; {exc}"
             return self.reject(
                 http.HTTPStatus.BAD_REQUEST,
-                f"Failed to open a WebSocket connection: {exc}.\n",
+                f"Failed to open a WebSocket connection: {exc_str}.\n",
             )
         except Exception as exc:
             # Handle exceptions raised by user-provided select_subprotocol and