Recommend dumping database backups with "--compress --output X" instead of "| gzip > X"

Summary:
Fixes T13304. Shell pipes and redirects do not have robust behavior when errors occur. We provide "--compress" and "--output" flags as robust alternatives, but do not currently recommend their use.

• Recommend their use, since their error handling behavior is more robust in the face of issues like full disks.
• If "--compress" is provided but won't work because the "zlib" extension is missing, raise an explicit error. I believe this extension is very common and this error should be rare. If that turns out to be untrue, we could take another look at this.
• Also, verify some flag usage sooner so we can exit with an error faster if you mistype a "bin/storage dump" command.

Test Plan: Read documentation, hit affected error cases, did a dump and spot-checked that it came out sane looking.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13304

Differential Revision: https://secure.phabricator.com/D20572