storage: handle errors in read stream

[stephenplusplus]

Fixes #724
Fixes #745 (comment)

@beshkenadze caught a bug in how we were handling responses from a file.createReadStream:

If not authorized request (403 error), then there is a exception. ;)

/Users/akira/Projects/My/ReviewHub/src/node_modules/gcloud/lib/storage/file.js:552
        res.headers['x-goog-hash'].split(',').forEach(function(hash) {
                                  ^
TypeError: Cannot read property 'split' of undefined
    at DestroyableTransform.<anonymous> (/Users/akira/Projects/My/ReviewHub/src/node_modules/gcloud/lib/storage/file.js:552:35)
    at DestroyableTransform.emit (events.js:107:17)
    at DestroyableTransform.<anonymous> (/Users/akira/Projects/My/ReviewHub/src/node_modules/gcloud/node_modules/stream-forward/index.js:31:26)
    at DestroyableTransform.<anonymous> (/Users/akira/Projects/My/ReviewHub/src/node_modules/gcloud/node_modules/stream-forward/node_modules/on-everything/index.js:12:17)
    at DestroyableTransform.obj.(anonymous function) [as emit] (/Users/akira/Projects/My/ReviewHub/src/node_modules/gcloud/node_modules/stream-forward/node_modules/stubs/index.js:28:10)
    at StreamCache.<anonymous> (/Users/akira/Projects/My/ReviewHub/src/node_modules/gcloud/node_modules/stream-forward/index.js:31:26)
    at StreamCache.<anonymous> (/Users/akira/Projects/My/ReviewHub/src/node_modules/gcloud/node_modules/stream-forward/node_modules/on-everything/index.js:12:17)
    at StreamCache.obj.(anonymous function) [as emit] (/Users/akira/Projects/My/ReviewHub/src/node_modules/gcloud/node_modules/stream-forward/node_modules/stubs/index.js:28:10)
    at Request.<anonymous> (/Users/akira/Projects/My/ReviewHub/src/node_modules/gcloud/node_modules/stream-forward/index.js:31:26)
    at Request.<anonymous> (/Users/akira/Projects/My/ReviewHub/src/node_modules/gcloud/node_modules/stream-forward/node_modules/on-everything/index.js:12:17)

@beshkenadze - #745 (comment)

In a recent commit, I mistakenly thought we could safely remove util.handleResp from the request stream's complete event. That was wrong, but thankfully, we're still on master without having made a release yet!

This adds that back in, so that errors are properly handled, as well as tests to cover that case.

And while I was at it, I fixed up #724, which exposes the abort function from retry-request. We use this to manually abort the request stream in the event of an error.

[callmehiphop]

Thanks!