Make the Diffusion warning about "svnlook" and PATH more clear

Summary:
See https://discourse.phabricator-community.org/t/display-error-on-the-status-page-for-svn-repos/2443 for discussion.

The UI currently shows a misleading warning that looks like "found svnlook; can't find svnlook".

It actually means "found svnlook, but when Subversion wipes PATH before executing commit hooks, we will no longer be able to find it".

Test Plan:

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20210