review: Request Component Error Handling Improvements

[runspired]

on a completely different note: I've been thinking through some feedback (both explicit and implicit) from @mkszepp RE the <Request /> component. Some unorganized thoughts:

• enforcing error blocks by rethrowing if none is present is good. Throwing in a way that crashes the app is bad.
• we should consider erring sooner (to ensure error block is defined) and potentially a lint rule around ensuring an error block is present
• we should consider a global-ish error capture fallback (I dislike this for a lot of reasons)
• we should consider rethrowing the error async so that it does not crash the app and can be caught by unhandled-exception instrumentation
• we should consider pushing for error boundaries in ember apps so that we can safely throw in templates and allow handling to occur at a point higher up of the application's choosing
• we should consider having a fallback rendered output for the error if no error block is present

fallback would be roughly

<pre error-trace>{{error.message}}\n\n{{error.stack}}</pre>