Need distinct error type for errors caused by wait

[jclark]

Using wait on a value of type future<T> can result in an error; this happens if you wait for the same future more than once.

AFAIR, this and commit are the only cases where the language generates an error (as opposed to a panic). For commit we already define a distinct error type.

This relates to #134.

[jclark]

Another case where the language generates an error is with json and lax typing.