Better error messages on close

[jackkleeman]

This brings more in line with the go impl; exec manager tracks if it has exited and won't try to send stuff if it has, evaluators can close themselves at which point they will throw sensible errors if you try to use them.

[jasongwartz]

Looks like we've got tests hanging:

 PASS e2e/only_primitives/primitives.test.ts
  E2E of config with only primitive values
    ✓ can generate TypeScript sources and load valid values (291 ms)
    ✓ can generate TypeScript sources but error on evaluating invalid values: missing a required property (225 ms)
    ✓ can generate TypeScript sources but error on evaluating invalid values: property is of the wrong type (213 ms)
    ✓ can generate TypeScript sources but error on evaluating invalid values: property fails validation constraint (215 ms)
Test Suites: 1 passed, 1 total
Tests:       4 passed, 4 total
Snapshots:   0 total
Time:        2.961 s
Ran all test suites.
Jest did not exit one second after the test run has completed.
'This usually means that there are asynchronous operations that weren't stopped in your tests. Consider running Jest with `--detectOpenHandles` to troubleshoot this issue.

The latter three are test cases where Pkl itself will error and exit non-zero. The fact that they're succeeding means an error must be being thrown (as expected), but it looks like this change is causing something to hang open.

[jackkleeman]

ah, damn. will work on it. nice tests!