Don't wait until benchmark is finished to show contents of standard error

[Warbo]

Stderr can contain information that the user might want to see during a particular benchmark run (an obvious example is a progress meter). Whilst asv does have an option to show the contents of stderr, it doesn't do so during the run; instead, it's gathered up and only displayed after that benchmark is finished.

It would be nice if stderr were given out as it's received, rather than waiting.

[HaoZeke]

Closed by #1262