While I appreciate a need for "Error Logging as a Service", I really wish all of these kind of tools were open and could configured for logging back to our own servers without needing to access anything else in the "cloud". Building LoB web apps that are often accessed from heavily firewalled networks makes these tools, unfortunately, useless.
Easy to deploy internally. Python server, client libs for basically everything, plugins to chat apps, issue trackers, you name it: https://github.com/getsentry/sentry
I'm a fan of Sentry, and their hosted service is very affordable. The fact you can push data to it on your own terms is great. I use it in combination with Logstash as an output, so any logged errors reaching Logstash are automatically piped to Sentry.
As a bonus, should you wind up deploying Sentry internally, you need only update the API host in Logstash.
We use Sentry a lot at Justcoin. Frontend, backend, workers, everywhere. It doesn't look great and their support is a bit slow at times, but it works and is affordable for startups.
Do you need open source or are you OK running something proprietary? If the latter is an option, Rollbar (https://rollbar.com) can be deployed on-premise.
