[CI][Crossbow] Nightly build email should distinguish between new failures and still failing builds

[asfimport]

It would help with triaging the nightly build status if it were more readily visible which builds broke today vs. are still broken (and were triaged yesterday).

Reporter: Neal Richardson / @nealrichardson

_{Note: This issue was originally created as ARROW-7191. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
cc @bkietz

[assignUser]

http://crossbow.voltrondata.com