Fix missing meta files breaking Bevy on itch (#19268)

# Objective

Fixes #19029 (also maybe sorta #18002, but we may want to handle the SPA
issue I outlined there more gracefully?)


## Solution

The most minimal / surgical approach I could think of, hopefully
cherry-pickable for a point release.

It seems that it's not *entirely* crazy for web services to return 403
for an item that was not found. Here's an example from [Amazon
CloudFront
docs](https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/http-403-permission-denied.html#s3-origin-403-error).
If it is somewhat common for web services to behave this way, then I
think it's best to also treat these responses as if they were "not
found."

I was previously of the opinion that any 400 level error "might as well"
get this treatment, but I'm now thinking that's probably overkill and
there are quite a few 400 level statuses that would indicate some
problem that needs to be fixed, and interpreting these as "not found"
might add confusion to the debugging process.

## Testing

Tested this with a web server that returns 403 for requests to meta
files.

```bash
cargo run -p build-wasm-example -- --api webgl2 sprite && \
open "http://localhost:4000" && \
python3 test_403.py examples/wasm
```
`test_403.py`:
```python
from http.server import HTTPServer, SimpleHTTPRequestHandler
import os
import sys


class CustomHandler(SimpleHTTPRequestHandler):
    def do_GET(self):
        if self.path.endswith(".meta"):
            self.send_response(403)
            self.send_header("Content-type", "text/plain")
            self.end_headers()
            self.wfile.write(b"403 Forbidden: Testing.\n")
        else:
            super().do_GET()


if __name__ == "__main__":
    if len(sys.argv) != 2:
        print(f"Usage: {sys.argv[0]} <directory>")
        sys.exit(1)

    os.chdir(sys.argv[1])

    server_address = ("", 4000)
    httpd = HTTPServer(server_address, CustomHandler)
    httpd.serve_forever()
```

---------

Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: Ben Frankel <ben.frankel7@gmail.com>
Co-authored-by: François Mockers <francois.mockers@vleue.com>

crates/bevy_asset/src/io/wasm.rs

@@ -81,7 +81,10 @@ impl HttpWasmAssetReader {
                 let reader = VecReader::new(bytes);
                 Ok(reader)
             }
-            404 => Err(AssetReaderError::NotFound(path)),
+            // Some web servers, including itch.io's CDN, return 403 when a requested file isn't present.
+            // TODO: remove handling of 403 as not found when it's easier to configure
+            // see https://github.com/bevyengine/bevy/pull/19268#pullrequestreview-2882410105
+            403 | 404 => Err(AssetReaderError::NotFound(path)),
             status => Err(AssetReaderError::HttpError(status)),
         }
     }