Support Fastboot environment

[davewasmer]

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It would be great if we could support Rollbar in a Fastboot environment. I'm thinking we could import a shim that wraps Node's process.onUncaughtException in a window.onerror interface, so Rollbar could be imported (since it relies on that interface) and could report errors from Fastboot.

Open questions:

• [ ] Is this the right approach in Fastboot? Does it make sense to mock out window.onerror?
• [ ] Are there any concerns with the multiple application instance environment in Fastboot? Are there multiple logger instances? Do Rollbar reports need to distinguish between them?
• [ ] What additional metadata (if any) should be included to indicate the error comes from Fastboot?

[davewasmer]

After some discussion on the #-fastboot channel on slack, seems like it might be worth importing the Node version of Rollbar's library in a fastboot environment. Especially since the rollbar snippet that we inject actually builds a script tag and injects that to load the full lib - all stuff that is bound to get fouled up in Fastboot.