Provide a assets.fileExists() function or make assets.serve() throw

[enyo]

I want to render my own 404 pages, but Assets doesn't give me a clean way to do that.

It would be great if one of those could be implemented:

1. an assets.fileExists() function, so I can check if the file exists
2. a way to call assets.serve() so it throws an exception instead of responding with 404 itself.

What I'm doing to get this working is:

try {
  await gae.context.assets.read();
  gae.context.assets.serve();
}
on gae.AssetError {
  // Render my own 404 page
}

which obviously is less than ideal.

[enyo]

If you tell me which version you'd prefer I can create a PR.

[sgjesse]

Having asset.exists([String path]) would be a good API. This only issue is that if the underlying file system is changed between a call to exists and server or read the builtin 404 could still be returned. However for a deployed server the content in build/web is not supposed to change.

Saying that build/web is not supposed to change can also make it possible to cache the result of exists calls and potentially what is read by server or read as well.