Fail fast when non-existent URL passed to docker build

This PR is to address issue talked in #115 
@smarterclayton @csrwng @mfojtik lemme know what do you think?

The other thing is, I'd like to extend our integration tests a little bit, as it was talked some time ago on IRC with possibility to test e.g. this as well. I'll working on it as well, unless told otherwise.
How much more complex does this script get before it should be written in a real programming language?

----- Original Message -----
> > @@ -14,6 +14,17 @@ if [ -n "$DOCKER_REGISTRY" ]; then
> >    TAG=$DOCKER_REGISTRY/$BUILD_TAG
> >  fi
> >  
> > +if [[ $DOCKER_CONTEXT_URL != "git://"* ]] && [[ $DOCKER_CONTEXT_URL !=
> > "git@"* ]]; then
> > +  URL=$DOCKER_CONTEXT_URL
> > +  if [[ $URL != "http://"* ]] && [[ $URL != "https://"* ]]; then
> > +    URL="https://"$URL
> > +  fi
> > +  curl -sf $URL > /dev/null
> 
> I think we also need to be able to handle redirect (`-L` option) and deal
> with invalid SSL certificates (?). Curl provides many useful options for
> retries/timeouts, worth to explore them as Dan mentioned.
> 
> ---
> Reply to this email directly or view it on GitHub:
> https://github.com/openshift/origin/pull/140/files#r17983782