[Blazor] Provide a better error message when an asset is not found because the environment is `Production` and the app has not been published.

[javiercn]

[SteveSandersonMS]

Isn't the issue when we're running in Production, not development? I thought we'd need to detect the case where, in Production, the SWA files aren't present on disk. Or am I misinterpreting?

[javiercn]

@SteveSandersonMS I tried to clarify things a bit.

[javiercn]

This now produces the following warning