Summary: See PHI1025. When you "Import Columns", we test if you're trying to import into a board that already has columns. However, this test is too broad (it incorrectly detects "proxy" columns for milestones as columns) and not user-friendly (it returns 400 instead of a readable error). Correct these issues, and refine some of the logic around proxy columns. Test Plan: - Created a project, A. - Created a milestone under that project. - Imported another project's columns to A's workboard. - Before change: Unhelpful 400. - After change: import worked fine. - Also, hit the new error dialogs and read through them. Reviewers: amckinley Reviewed By: amckinley Differential Revision: https://secure.phabricator.com/D19978