Misleading error message if wgsl import has bad path

## Bevy version

Relatively new: 499c978

## What you did

I have a wgsl shader which at the start has:

```wgsl
#import bevy_ui::ui_vertex_output::UiVertexOutput
#import bevy_pbr::rand_f
```

## What went wrong

When trying to use `rand_f`, an error happens:

```
2024-03-06T10:39:12.086331Z ERROR bevy_render::render_resource::pipeline_cache: failed to process shader:
error: required import 'bevy_pbr' not found
┌─ foo.wgsl:14:17
│
14 │ let noise = rand_f(&rng);
│ ^
│
= missing import 'bevy_pbr'
```

## Additional information

### Theories

While typing out this issue I realized my mistake.

The import should be

```wgsl
#import bevy_pbr::utils::rand_f
```

not

```wgsl
#import bevy_pbr::rand_f
```

### Suggested fix

Improve the error message to state that `bevy_pbr::rand_f` does not exist, not the current message which is misleading.