Add unit tests exercising each error code

[plaidfinch]

We want to make sure the errors we expect are the ones we get, so it would be good to add some expected-to-fail unit tests.

[pnwamk]

Commit 5dcb409 has partially addressed this (via unit tests in the file-echo-api unit tests), however, the below listed error codes could still use some manual checks.

• parse error (-32700)
• invalid request (-32600)
• internal error (-32603)
• unknown state id (20)

N.B., some of these error codes may be better tested directly (i.e., from a simple Haskell unit test) if the relevant portion of the python argo API is designed to only send correct, well-formed requests.