cmd/releasebot: check os.Stat error in loadGithubAuth
cmd/releasebot: check os.Stat error in loadGithubAuth

The error value returned from os.Stat call was not being checked.
This change fixes that.

Change-Id: I640c59275aa7adf21fa5ba52e1d114473ff3c82d
Reviewed-on: https://go-review.googlesource.com/103855
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build