Fix an issue where "Import Columns" could fail on a board for a project with milestones

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