(PUP-3322) Add debugging to appdmg provider

I was trying to install Scratch[1] using boxen[2]. The appdmg isn’t in
the expected format, so this failed. I had to poke around a bit to
understand why that was happening. This change captures that knowledge.

[1] http://scratch.mit.edu/scratch_1.4/
[2] https://boxen.github.com/