Fix some Windows EDITOR issues

Summary:
See D2955, D2956. Fix some more issues:

• We're escaping in a way that makes it easier to specify EDITOR but prevents you from specifying flags.
• Catch complete proc_open() failures and fail less mysteriously.
• Don't use +line on Windows since it doesn't work with most editors.

Test Plan: Created this diff on Windows.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1309

Differential Revision: https://secure.phabricator.com/D2957