AutoPPA should fail if debian/ doesn't exist

I was playing around with using autoppa on a bzr loom. I had committed the upstream source in one thread, and the debian/ directory in another thread. However, I had not recorded the loom.

The upshot of this was that when autoppa branched the source, the branch didn't include debian/, which led to build failure with a very confusing error message that didn't make any sense until after some Googling.

AutoPPA should probably validate that debian/ exists in the branch, and error out the entire process if it's missing. Possibly with some cmdline override if some strange use-case needs it.