Friendlier error when GitHub repo doesn't exist

[patrickhulce]

First off, thanks for this project it's a legitimate life saver!

I recently ran into the following issue. Running semantic-release-cli init I get the following stack trace when my package.json repo URL is incorrect and points to nothing.

Would be nice to have a "check your package.json points to a valid GitHub repo" or something instead :)

ERR! semantic-release TypeError: Cannot read property 'slug' of undefined
ERR! semantic-release     at /Users/patrick/.nvm/versions/node/v7.5.0/lib/node_modules/semantic-release-cli/dist/lib/travis.js:90:68
ERR! semantic-release     at Generator.next (<anonymous>)
ERR! semantic-release     at step (/Users/patrick/.nvm/versions/node/v7.5.0/lib/node_modules/semantic-release-cli/dist/lib/travis.js:114:191)
ERR! semantic-release     at /Users/patrick/.nvm/versions/node/v7.5.0/lib/node_modules/semantic-release-cli/dist/lib/travis.js:114:361
ERR! semantic-release     at process._tickCallback (internal/process/next_tick.js:103:7)