Give less scary error messages when a containerised command fails

This affects errors encountered at build time and at system-integration time.

New errors look like this:

    ERROR: Command failed: baserock/system-integration/02-install-gerrit-gerrit-installation-binaries-misc-0000:
    Containerisation settings: {'mounts': (('dev/shm', 'tmpfs', 'none'), ('tmp', 'tmpfs', 'none')), 'mount_proc': True, 'root': '/var/tmp/staging/tmp1YQ2yN/minimal-system-x86_64-generic.inst'}
    Error output:
    + install -D /usr/share/gerrit/gerrit-2.9.war /home/gerrit2/gerrit/gerrit-2.9.war -o gerrit2 -g gerrit2 -m 644
    install: can't change ownership of /home/gerrit2/gerrit/gerrit-2.9.war: Operation not permitted

Previously the error message would have been this:

    Command failed: unshare --mount -- sh -ec.
    mount --make-rprivate /
    root="$1"
    shift
    while true; do
        case "$1" in
        --)
            shift
            break
            ;;
        *)
            mount_point="$1"
            mount_type="$2"
            mount_source="$3"
            shift 3
            path="$root/$mount_point"
            mount -t "$mount_type" "$mount_source" "$path"
            ;;
        esac
    done

    exec "$@"
     - /var/tmp/staging/tmppeA1Iw/gerrit-x86_64.inst/ dev/shm tmpfs none tmp tmpfs none -- linux-user-chroot --chdir . --mount-proc proc
    /var/tmp/staging/tmppeA1Iw/gerrit-x86_64.inst/ baserock/system-integration/02-install-gerrit-gerrit-installation-binaries-misc-0000
    + install -D /usr/share/gerrit/gerrit-2.9.war /home/gerrit2/gerrit/gerrit-2.9.war -o gerrit2 -g gerrit2 -m 644
    install: can't change ownership of /home/gerrit2/gerrit/gerrit-2.9.war: Operation not permitted

1 files changed, 16 insertions, 0 deletions

diff --git a/morphlib/util.py b/morphlib/util.py
index 6f735387..736af92e 100644
--- a/morphlib/util.py
+++ b/morphlib/util.py
@@ -15,11 +15,13 @@
 
 import contextlib
 import itertools
+import logging
 import os
 import re
 import subprocess
 import textwrap
 
+import cliapp
 import fs.osfs
 
 import morphlib
@@ -626,3 +628,17 @@ def containerised_cmdline(args, cwd='.', root='/', binds=(),
     cmdargs.append(root)
     cmdargs.extend(args)
     return unshared_cmdline(cmdargs, root=root, **kwargs)
+
+
+def error_message_for_containerised_commandline(
+        argv, err, container_kwargs):  # pragma: no cover
+    '''Return a semi-readable error message for a containerised command.'''
+
+    # This function should do some formatting of the container_kwargs dict,
+    # rather than just dumping it in the error message, but that is better than
+    # nothing.
+
+    return 'Command failed: %s:\n' \
+           'Containerisation settings: %s\n' \
+           'Error output:\n%s' \
+           % (' '.join(argv), container_kwargs, err)