[dock] Make raise_on_errors a member and use it in _exec

This allows to be coherent when running a command and docker is not
installed.

For example _discover_environment will fail even if raise_on_errors is
set to False if docker is not installed. This patch fixes that.
7 jobs for improve_raise_on_errors_handling in 42 minutes and 54 seconds (queued for 7 seconds)
Status Name Job ID Coverage
  Build
passed build_linux_27 #137714
docker

00:05:21

60.0%
passed build_linux_36 #137715
docker

00:05:20

60.0%
passed build_macosx_27 #137736
macosx

00:08:03

60.0%
passed build_macosx_36 #137717
macosx

00:08:54

60.0%
failed build_macosx_27 #137716
macosx

01:30:00

 
  Docker
passed docker_linux_27 #137718
docker-build

00:25:57

70.0%
passed docker_linux_36 #137719
docker-build

00:25:35

70.0%