acme/autocert: return error if Prompt not set

Without this, autocert will panic with an unhelpful nil pointer inside
the acme client.

Reorganized the test suite's ACME server stub creation, as I needed to
be able to stand up a test where GetCertificate was expected to fail.

Change-Id: Ie5e19c6e7766b4578c9b3c16789d7b27bd3be163
Reviewed-on: https://go-review.googlesource.com/40951
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
2 files changed