Show a useful error message when no credentials file has been found

[longouyang]

Currently, we can partially run nosub upload without credentials but we eventually hit an uninformative timeout error:

Running on sandbox
How many assignments do you want to run?
> 3
How long do you want to run the HIT?
You can answer in seconds, minutes, hours, days, or weeks and you can always add more time using nosub add.
> 2 days
Cost will be $1.7999999999999998
(node:75915) UnhandledPromiseRejectionWarning: Error: connect ETIMEDOUT 169.254.169.254:80
    at TCPConnectWrap.afterConnect [as oncomplete] (net.js:1097:14)
(node:75915) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). (rejection id: 1)
(node:75915) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

We should check for credentials earlier