tests: Don't do SystemExit when docker fails, works badly with xdist.
tests: Don't do SystemExit when docker fails, works badly with xdist.

Instead of displaying the info from the exception, it displays
INTERNALERROR and all hell breaks loose. Try normal exception instead
to get more info, and more predictable behavior.

Changelog: None

Signed-off-by: Kristian Amlie <505e66ae45028a0596c853559221f0b72c1cee21@northern.tech>

GregorioDiStefano/integration,pasinskim/integration,pasinskim/integration,GregorioDiStefano/integration,pasinskim/integration