Unhelpful error message on register-ci

Hey.
I got this error

```
CI Summary for scikit-learn/scikit-learn-feedstock (can take ~30s):
* scikit-learn/scikit-learn-feedstock already enabled on travis-ci
* scikit-learn/scikit-learn-feedstock enabled on CircleCI
* scikit-learn/scikit-learn-feedstock already enabled on appveyor
Traceback (most recent call last):
File "/home/andy/anaconda3/bin/conda-smithy", line 11, in
load_entry_point('conda-smithy==1.2.0', 'console_scripts', 'conda-smithy')()
File "/home/andy/anaconda3/lib/python3.5/site-packages/conda_smithy/cli.py", line 236, in main
args.subcommand_func(args)
File "/home/andy/anaconda3/lib/python3.5/site-packages/conda_smithy/cli.py", line 166, in __call__
ci_register.appveyor_configure(owner, repo)
File "/home/andy/anaconda3/lib/python3.5/site-packages/conda_smithy/ci_register.py", line 149, in appveyor_configure
raise ValueError(response)
ValueError:
```

when trying to push an rc into our own channel.
I guess it's a permission problem on some level, but it would be nice to have a better error message.