Missing shader wsgl file doesn't lead to panic

## Bevy version
0.8

## What you did

Copy shader instancing example code but didn't put the instancing.wgsl file in asset/shaders.

## What went wrong

- what were you expecting? instance shading
- what actually happened? nothing
- what would you like to happen? A big fat error message pointing out that the shader file was not found at this location.

I'm sure I won't be the last idiot to not copy the shader file, would be great to have ideally a better error message or to point out at the top of the example file the additional asset files that the example depends on (or both).

Start with the shader instancing example and trace how it loads asset/shaders/instancing.wgsl when that file is absent. Run the example without the shader file to observe the current behavior. Done means the missing dependency produces a clear error pointing to the expected shader location, or the example clearly identifies its required asset files.

Written by the indexing model from the issue text.