Append '\n' character to nanobench error message

This prevents the error message from inserting itself on a line before
a bench result, which can mess up parsing.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1540923003

Review URL: https://codereview.chromium.org/1540923003
1 file changed