Vertex names the variable when its credentials file is missing

When using Claude through Google Vertex, a missing credentials file named by GOOGLE_APPLICATION_CREDENTIALS, or its lowercase spelling, now gets its own error. It says which variable named the file and the path that was not found. The "Service account key file not found" error now appears only when the missing file is the configured service account key file. Before, any missing file under service account sign-in gave that message.