Improve error reporting for shell commands

Currently when a shell command fails, only the stdout is included
in the error message.  In some cases this is not useful because
the relevant error information is in stderr.  This change adds
stderr to the error message so a user may have enough information
to resolve the problem.

Change-Id: I595f7fd1a398c453ddc7757c551625c5bfa88d3a
Fixes: rhbz#1031786
Merge "Improve error reporting for shell commands"