Display chroot names and details

Add option '-a' to operate on all chroots and '-l' to list croutonversion details.
Call `"$BINDIR/edit-chroot" -c "$CHROOTS" -a` here, shorten message to "Valid chroots:", and put the list on the next line.
Note that you can have newlines in the error message, so you can do something like

```
    error 1 "\
$CHROOTSRC does not exist; cannot update.
This is the second line of my error
`echo "insert output of command on this line"`"