"unable to verify team member from database" error should include the failed team member

[anp]

This is an uninformative error message:

thread 'teams::test::team_members_exist' panicked at 'unable to verify team member from database.
if you're running this for tests, make sure you've pulled github users from prod: DieselError(NotFound)', libcore/result.rs:983:5

Originally posted by @anp in https://github.com/anp/rfcbot-rs/pull/247#issuecomment-435399127

[Centril]

I'm sorry I haven't been able to develop anything on rfcbot lately as much as I wanted to :( but you know, RFCs need to be written and stuff ;)

[anp]

Oh my, if you have to apologize then I have transgressed beyond comprehension :P. I had thought I wrote that error message when I wrote the original message, but it was still a good idea to change my wording. Sorry <3.