add better error message when network lock is held

[mitchellwrosen]

This PR adds a better error message when attempting to acquire the network lock while it is held.

I think this partially addresses #190, though we could/should also add documentation to execute to call out this behavior.

To do:

• [ ] Decide on an exception type. This currently uses fail in IO, so IOException. I find this kind of nasty and old-school, but this exception shouldn't really ever be caught and handled in any particular way.
• [ ] Nail down exact wording of error message
• [ ] Update execute documentation (and are there other common ways this can occur?)