store: fix panic error in auth

[stolowski]

Fix panic on DNS error in auth, caused by shadowing of err. Added a couple extra tests. Store requests do not have this bug, only auth.

[zyga]

+1

[pedronis]

thank you

[stolowski]

Hmm, looks like the test I added fails with 503 for some reason when run on Travis, which is different error that I get locally and expect in the test:

store_test.go:2963:
    c.Assert(err, ErrorMatches, `Post http://nonexistingserver909123.com/updates/: dial tcp: lookup nonexistingserver909123.com on .*: no such host`)
... error string = "cannot query the store for updates: got unexpected HTTP status code 503 via POST to \"http://nonexistingserver909123.com/updates/\""
... regex string = "Post http://nonexistingserver909123.com/updates/: dial tcp: lookup nonexistingserver909123.com on .*: no such host"