More verbose error handling in dabuildsys

[jdreed]

In particular, there's a lot of subprocess.check_output() calls, and it's not always clear why something failed. Specifically, we should document where our assumptions are, and use try/catch, or better yet, custom Exceptions somewhere. For example:

Failed building firefox-extension: Command '['git', 'rev-parse', 'refs/tags/10.2.2^{}']' returned non-zero exit status 128

Could better be written as:

Failed building firefox-extension: Command '['git', 'rev-parse', 'refs/tags/10.2.2^{}']' returned non-zero exit status 128
Did you forget to tag the upstream version?

And

dabuildsys.common.BuildError: File /home/builder/build/orig-tarballs/firefox-extension-10.2.2.tar.gz already exists, aborting

could have "Did you mean to run this with -P?" appended to it.