Catch exception if travis isn't set up

[gforsyth]

Just caught this when I tried to regenerate the deploy key without first enabling travis

Key generation went fine but then requests barfed when it couldn't talk to travis

Traceback (most recent call last):
  File "/home/gil/anaconda/bin/doctr", line 9, in <module>
    load_entry_point('doctr==1.1.1+42.g9c5359b', 'console_scripts', 'doctr')()
  File "/home/gil/anaconda/lib/python3.5/site-packages/doctr-1.1.1+42.g9c5359b-py3.5.egg/doctr/__main__.py", line 210, in main
    return process_args(get_parser())
  File "/home/gil/anaconda/lib/python3.5/site-packages/doctr-1.1.1+42.g9c5359b-py3.5.egg/doctr/__main__.py", line 96, in process_args
    return args.func(args, parser)
  File "/home/gil/anaconda/lib/python3.5/site-packages/doctr-1.1.1+42.g9c5359b-py3.5.egg/doctr/__main__.py", line 153, in configure
    encrypted_variable = encrypt_variable(b"DOCTR_DEPLOY_ENCRYPTION_KEY=" + key, build_repo=build_repo)
  File "/home/gil/anaconda/lib/python3.5/site-packages/doctr-1.1.1+42.g9c5359b-py3.5.egg/doctr/local.py", line 45, in encrypt_variable
    r.raise_for_status()
  File "/home/gil/anaconda/lib/python3.5/site-packages/requests/models.py", line 837, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 404 Client Error: Not Found for url: https://api.travis-ci.org/repos/drdoctr/doctr/key

We can just catch this exception and gently remind the user to go to travis and enable the repository

[asmeurer]

It will also give this error if the repo doesn't exist, so we should check that too (ideally before generating the ssh key).