Print the name of the actual missing dependency in async.auto

[jahraphael]

Hi, I'm not sure if there were specific guidelines to go through before submitting a PR, so apologies if I've missed a step. Now on to the PR:

We've been making use of async.autoInject as a light dependency injection framework, and whenever we'd make a mistake in a tasks dependencies we'd end up with a time-consuming task to track down what dependency async was failing on, because the error message wasn't giving enough to go on.

This PR simply adds the actual failed dependency to the error message to trivialize the work of tracking down missing dependencies.

[aearly]

Can you tweak the test so it checks for the dependency name as well?

[jahraphael]

Sure, no problem.

I've pushed up changes that check within the auto prevent dead-locks due to inexistant dependencies test whether the error contains the missing dependency name (/dependency `noexist`/).

[megawac]

Thanks a bunch!