Make code that parses callback errors more tolerant of message format.

JAX is switching from pybind11 to nanobind, and nanobind formats this
error differently, with the traceback preceding the EqxRuntimeError
message. If we change the "At:" split to accept zero or more "At:"
pieces, things work as intended with both versions.

equinox/_jit.py

@@ -212,7 +212,7 @@ def __call__(self, /, *args, **kwargs):
  (msg,) = e.args
  if "EqxRuntimeError: " in msg:
  _, msg = msg.split("EqxRuntimeError: ", 1)
- msg, _ = msg.rsplit("\n\nAt:\n", 1)
+ msg, *_ = msg.rsplit("\n\nAt:\n", 1)
  msg = msg + _eqx_on_error_msg
  e.args = (msg,)
  if jax.config.jax_traceback_filtering in ( # pyright: ignore