lockdown() should bail if it cannot get a direct eval

[warner]

Agoric/agoric-sdk#1182 fixed a problem in the metering transform which broke the direct eval that SES-shim needs to build the proxy-scoped evaluator (turning it into an indirect eval). The symptom was subtle: added endowments went missing, but evaluation worked fine otherwise.

Should we add code to lockdown() to assert that it has access to a functional direct eval, and abort if it does not? The SES shim is, of course, entirely dependent upon the environment that loads it, but if a vetted shim were to unintentionally break direct eval, it might be better to halt than to produce a subtly-broken SES environment.

Originally posted by @warner in Agoric/agoric-sdk#1182 (comment)

[erights]

Yes. It should bail with a clear diagnostic