handle error from closing GCS writer

[robwhitby]

Sometimes writing the version file to GCS can silently fail and the file is empty after a put. The error only surfaces on subsequent get or check (see #125).

This change no longer ignores errors returned when closing the GCS writer, as the library docs state:

Since writes happen asynchronously, Write may return a nil error even though the write failed (or will fail). Always use the error returned from Writer.Close to determine if the upload was successful.

[taylorsilva]

LGTM, thanks for the PR!