Ensure that sendMessage's callback is called.

Also: unchecked errors are now also reported to the extension's console.
Unchecked errors were silenced before because the internal disconnect
handler accessed chrome.runtime.lastError.

BUG=439780
TEST=browser_tests --gtest_filter=ExtensionApiTest.MessagingCrash

Review URL: https://codereview.chromium.org/1567423002

Cr-Commit-Position: refs/heads/master@{#368469}
3 files changed