Make ReactWebWorker-test.js less flaky.

Two improvements: make sure we set `done = true` when an error message is
received, so that the test output contains the error message instead of
eventually timing out and displaying nothing useful; and increase the
timeout for this particular test from 5000ms (the default) to 20000ms.
Merge pull request #1099 from benjamn/make-ReactWebWorker-test-less-flaky

Make ReactWebWorker-test.js less flaky.