Only print stdout/stderr if we've got an unexpected output

Only print stdout/stderr if we've got an unexpected output

TBR=ricow

Committed: https://code.google.com/p/dart/source/detail?r=17767

[kustermann, 2013-01-29 15:43:15]

I accidentally introduced the printing of stdout/stderr information on non-zero
exit code instead of unexpected output.
This CL fixes that.

[ahe, 2013-01-29 18:27:43]

LGTM!