include sources in shader validation error (#3724)

## Objective

When print shader validation error messages, we didn't print the sources and error message text, which led to some confusing error messages.

```cs
error: 
   ┌─ wgsl:15:11
   │
15 │     return material.color + 1u;
   │           ^^^^^^^^^^^^^^^^^^^^ naga::Expression [11]
```

## Solution

New error message:
```cs
error: Entry point fragment at Vertex is invalid
   ┌─ wgsl:15:11
   │
15 │     return material.color + 1u;
   │           ^^^^^^^^^^^^^^^^^^^^ naga::Expression [11]
   │
   = Expression [11] is invalid
   = Operation Add can't work with [8] and [10]
```

crates/bevy_render/src/render_resource/pipeline_cache.rs

@@ -464,15 +464,22 @@ fn log_shader_error(source: &ProcessedShader, error: &AsModuleDescriptorError) {
                 let config = term::Config::default();
                 let mut writer = term::termcolor::Ansi::new(Vec::new());
 
-                let diagnostic = Diagnostic::error().with_labels(
-                    error
-                        .spans()
-                        .map(|(span, desc)| {
-                            Label::primary((), span.to_range().unwrap())
-                                .with_message(desc.to_owned())
-                        })
-                        .collect(),
-                );
+                let diagnostic = Diagnostic::error()
+                    .with_message(error.to_string())
+                    .with_labels(
+                        error
+                            .spans()
+                            .map(|(span, desc)| {
+                                Label::primary((), span.to_range().unwrap())
+                                    .with_message(desc.to_owned())
+                            })
+                            .collect(),
+                    )
+                    .with_notes(
+                        ErrorSources::of(error)
+                            .map(|source| source.to_string())
+                            .collect(),
+                    );
 
                 term::emit(&mut writer, &config, &files, &diagnostic).expect("cannot write error");
 
@@ -490,3 +497,25 @@ fn log_shader_error(source: &ProcessedShader, error: &AsModuleDescriptorError) {
         }
     }
 }
+
+struct ErrorSources<'a> {
+    current: Option<&'a (dyn std::error::Error + 'static)>,
+}
+
+impl<'a> ErrorSources<'a> {
+    fn of(error: &'a dyn std::error::Error) -> Self {
+        Self {
+            current: error.source(),
+        }
+    }
+}
+
+impl<'a> Iterator for ErrorSources<'a> {
+    type Item = &'a (dyn std::error::Error + 'static);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let current = self.current;
+        self.current = self.current.and_then(std::error::Error::source);
+        current
+    }
+}