storage: fix credentials issue.

Previously, if a user wanted to generate a signed URL, there was
no guarantee a credentials object would have already been
generated. By itself, generating a signed URL doesn't require a
network connection, thus the uncertainty that we would have
already fetched a token. This change requires a valid credentials
object to exist before returning the `credentials` object.
Merge pull request #164 from stephenplusplus/connection

storage: fix credentials issue.