Observers causes a panic in scenes

## Bevy version

e155fe1

## What you did

Create a simple scene with an observer.

## What went wrong

Scene spawn system causes a crash caused by the observer.

## Additional information

Observers are a component that doesn't implement Reflect, thus causing the crash.

PS: Scene spawning should never cause a crash in the first place, either building the scene should be fallible or the scene spawning ignored with a warning.

Start by reproducing the crash with a simple scene containing an observer, then inspect the scene-spawning path and how non-Reflect components are handled. Done means scene spawning no longer panics; the issue suggests either a fallible build or a warning when the observer cannot be represented.

Written by the indexing model from the issue text.