[CKCT] Failures in deployment stage won't get printed

[Po-Hsu Lin]

This bug was filed to follow up bug 1828769

It seems that when there something failed during the deployment stage, aka the steps in lib/target.py:
        s.mainline_firmware_hack()
        s.install_python_minimal()
        s.install_required_pkgs()

the error won't get printed to the tester, this makes it hard to debug issues like 1828769

Here is an output example when it tries to install sbsigntools in s.install_required_pkgs() on a s390x KVM (which does not have this package)

MAAS Testing Selected
        Allocating...
        Deploying...
        Deployed
maas maas2 s2lp6g003
waiting
waited
 . Fixup /etc/hosts
 . Fixup /etc/apt/sources.list
 . Enabling Proposed
 . Enabling Src
 . Enabling PPA
 . Disabling Periodic APT Updates
 . Installing python-minimal
 . Installing Required Packages
 . Installing Kernel Meta Package
 . Rebooting for installed Kernel meta package
 . Verifying the kernel signature
 . Verifying the running kernel version
 . Verifying base install
           series: bionic
             arch: s390x
        uname -vr: 4.15.0-1008-fips #9-Ubuntu SMP Wed May 22 13:14:14 UTC 2019
 . That's All Folks!
-------------------------------------------------------------------------------------------------------

As the package is not available for s390x, this "Installing Required Packages" is actually failed.

It will be great if the error can be printed out.