csv: Figuring out which valid CSV line fails account create is hard

When CSV was supplied in valid form but was going to create duplicate
or otherwise invalid accounts per ManagedSubject validator the
resulting display for users was confusing and unhelpful.

This commit highlight the specific entry that was being rejected
along with why. We also provide an additional helpful pointer to the
possibility that the account being created is a duplicate as this is
the major reason CSV upload fails at this point.

Closes #168.