diagnose_apk_bloat.py: fix error messages and simplify rev order.

  * Improve error messages related to supplying invalid rev or
--reference-rev args
  * Change diff classes to use "before" and "after" instead of a list
of dirs to make the code a little easier to follow
  * Fix Linux issues and make --cloud work for Linux

BUG=695182

Review-Url: https://codereview.chromium.org/2847243005
Cr-Commit-Position: refs/heads/master@{#468690}
2 files changed