Send Sentry message when sendEvent isn't defined

[ianb]

While we worked around sendEvent being suppressed sometimes (by unknown forces), the browser-send-event.js module mostly just keeps things moving. But we should send a Sentry message to try to understand why this might be happening. Best would be to send an event to the add-on which could then report to Sentry and include information about other add-ons that might be causing this.