Rely on the JavaScript spec to handle more module errors

This builds on tc39/ecma262#916, in which the
JavaScript specification remembers errors during module instantiation
and evaluation. That allows us to stop our error-prone approach of
doing bottom-up instantiation (which currently fails for cyclic graphs;
see #2629), and obviates the consequent need to propagate errors (which
is also buggy; see #2630). Finally, it makes certain edge cases during
evaluation nicer; see
#2595 (comment).

For background on why we originally went with a bottom-up approach, see
#1545. At the time we didn't seem to believe changing the JavaScript
spec was an option, but it's become increasingly clear it's the most
reasonable one.

Closes #2629. Closes #2630.

Author: domenic