Fail more softly if we can't execute "ps"

Summary: If, e.g., $PATH is broken we may not be able to run "ps". We'll explode pretty hard, currently. Instead, just show a harsher warning.

Test Plan: Changed "ps auxwww" to "psq", which doesn't exist on my system. Loaded page, got warning instead of explosion.

Reviewers: nathanws, vrana, btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D2624