The cannot-squash error message is unhelpful

The following discussion from !348 (merged) should be addressed:

• @aqualls started a discussion:

  From speaking with @fseifoddini, she got the dreaded "cannot squash" message. (Hey - one of us should spin this comment off into a follow-up MR - that's an error message I can help improve.)

  I've squashed some of the commits together to see if I can get this MR into a mergeable state.