Currently, when you run `start` twice with the same config, it will
start the PekkoManagement extension once and return the same `Uri`.
This is a convenient 'idempotence'. However, previously this would
silently ignore the second invocation if the parameters of the two
calls were different, leading to a race condition.

The change in this commit adds error handling so that if the second
invocation uses different parameters, it will fail the second call
with a sensible error message.