Clarify API authentication error, fixes #368 (#688)
Clarify API authentication error, fixes #368 (#688)

Error message is passed from utility method that creates a secure connection to
a Google API [doc link](https://godoc.org/google.golang.org/api/transport#NewHTTPClient)
GoogleCloudPlatform/compute-image-tools,adjackura/compute-image-tools,GoogleCloudPlatform/compute-image-tools,GoogleCloudPlatform/compute-daisy,adjackura/compute-image-tools,zmarano/compute-image-tools,GoogleCloudPlatform/compute-image-tools,GoogleCloudPlatform/compute-image-import,zmarano/compute-image-tools,adjackura/compute-image-tools,GoogleCloudPlatform/compute-daisy,zmarano/compute-image-tools,GoogleCloudPlatform/compute-image-import,GoogleCloudPlatform/compute-image-import