Downgrade CaskAlreadyInstalledError to a warning

Automatic installs - like thoughtbot's laptop script - fail when we
use Homebrew's onoe method.  Using opoo (to warn) will let the
scripts complete successfully when they try to install packages that
have already been installed.

Test for downgrading AlreadyInstalledError to warning
use "staged" instead of "installed" for Caskroom

refs: #6783
demote the Tap migration banner in FAQ.md
hide error outputs from plutil
basic docs for stub gpg stanza
convert adobe-air.rb to use `installer :script`

* remove `caskroom_only true`
* remove `postflight`
convert amazon-music.rb to use `installer :script`

* remove `caskroom_only true`
* remove `postflight`
use `installer :script` in private-internet-access

* hoist primary command out of `postflight` into `installer` artifact
* this Cask did not use the `caskroom_only` idiom
use installer :manual in adobe-creative-cloud
use installer :manual in backblaze
use installer :manual in baiduinput
use installer :manual in dbvisualizer
use installer :manual in default-folder-x
use installer :manual in google-nik-collection
use installer :manual in jdownloader
use installer :manual in little-snitch
use installer :manual in logmein-hamachi
use installer :manual in paye-tools
use installer :manual in ps3-media-server
use installer :manual in scout