Improve auth error for BSR remotes

[emcfarlane]

Users interacting with a remote BSR may see an auth error when using the env variable BUF_TOKEN. This is reported as:

Failure: the BUF_TOKEN environment variable is set, but is not valid.

To help users identify which remote was used, we now report the remote in the error message:

Failure: the BUF_TOKEN environment variable is not valid for buf.build.

This can help clarify which remote was used. As an example this can be caused when a modules dependencies are on a different BSR instance then the parent module.

[github-actions[bot]]

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