Bail out if architecture isn't set nor can be identified automatically

If `dpkg --print-architecture` isn't available for detecting the current
architecture (e.g. on non-Debian), and neither $ARCH is set nor `--arch
...` was specified, abort and inform user about it.

Closes: #173

grml-debootstrap

@@ -1155,6 +1155,12 @@ else
    ARCHCMD="--arch $ARCH"
    ARCHINFO=" (${ARCH})"
 fi
+
+if [ -z "${ARCH:-}" ] ; then
+  eerror 'Architecture neither set (environment variable ARCH), nor could be automatically identified (using dpkg).'
+  eerror 'Consider setting the --arch ... option.' ; eend 1
+  bailout 1
+fi
 # }}}
 
 # It is not possible to build amd64 on i686. {{{