Output URL of new incomplete review

If `rbt post` did create review but failed before completing it, user had to
find the incomplete review themselves (and discard or complete it manually).

This patch adds the URI to the error message so that remedy is less cumbersome.

Testing Done:
I have tested this on my local machine, where rbt fails to post commit,
reporting HTTP 400 / API error 207. (Previous review request #5825 from Stephen
Gallagher addresses that)

Reviewed at https://reviews.reviewboard.org/r/5826/