Fix TypeError when rawBody returns err

[dalgleish]

When raw-body returns an err, the callback will be called with an undefined string parameter.
This change logs some detail about the errpr provided by raw-body and rejects the promise.

[dalgleish]

Sorry, test are not passing.
I'll follow the contribution guidelines and maybe I can add a test for this case.

BTW, we get this in production. We're using perimeterx-node-express@3.4.0:

TypeError: Cannot read property 'toString' of undefined
    at /app/node_modules/perimeterx-node-core/lib/pxproxy.js:240:42
    at invokeCallback (/app/node_modules/raw-body/index.js:224:16)
    at done (/app/node_modules/raw-body/index.js:213:7)
    at IncomingMessage.onAborted 

[yaronschwimmer]

Hi @dalgleish
Thank you for your contribution. We'll review it and merge if everything looks right.