Inform if build information are not available

[agners]

Currently, when build information are missing the following error is printed:

[15:41:46] ERROR: amd64 not supported for this build

This is rather misleading. Print an error when the build information are not available.

[mdegat01]

So I see your point but one thing that's confusing me is that this file is actually optional. If build.json or build.yaml isn't provided then it simply uses latest as per here:
https://github.com/home-assistant/builder/blob/3e71d6f20f929b30ce69d6283eb7f2c468a4f8f5/builder.sh#L495-498

Therefore I wonder whether users would find this message more or less confusing? Users shouldn't see an error just for omitting build.json so when does this error actually occur and how do they fix it?

[agners]

If build.json or build.yaml isn't provided then it simply uses latest as per here:

Afaik, that is only true for the addon build type, which this PR doesn't change.

[pvizeli]

I agree, it's possible to build an add-on without build information. The CI should filter out not needed arch builds, so the error is normally not show if it's using our latest CI

[agners]

This fix is meant for manual builds.

I agree, it's possible to build an add-on without build information.

Correct, and add-on builds are not touched by this change.

So I guess it is fine then to merge this?