errors not reported unless verbose is set

In stack.sh output to stdout / stderr is explicitly dropped :

    if [[ "$VERBOSE" != "yes" ]]; then
        # Throw away stdout and stderr
        exec 1>/dev/null 2>&1
    fi

however the library routines e.g. start_glance use simple echo messages to report e.g. the failure of glance
to start.

Errors should be visible in non verbose mode, so either we should squash the stdout/stderror redirect to null, or
all routines that report errors should have echo replaced with echo_summary