Arcanist - stop arc land if local is ahead of remote

Summary: Fixes T4291. Also pht user-facing strings.

Test Plan:
made a branch foo off master. made a commit and a diff in foo. switched backed to master and cowboy committed some thing. went back to branch`foo`, did an arc land, and saw the error message. went back to master, did a git resert --hard HEAD^1, went back to branch foo, and then successfully arc landed.

also ran arc help land and things looked good

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4291

Differential Revision: https://secure.phabricator.com/D8738