DuckPAN: move prefix-appending into `_print_msg`

This may (or may not) make it easier to do more interesting things with
the various kinds of output.
Server: left-align ProgressBar.

Already seeing the problems with something which doesn't go through the
`emit` process.  It's still helpful, but it requires extra maintenance
effort when the assumptions change.
DuckPAN: bold all prefix lines, NOTE -> NOTICE

The intention is to have them stand out a bit, so this should help.
In the old case, it could blend in with the output if the user happens
to be using a chosen color as their base color, since there aren't
any obvious prefixes on the standard output. If people run with bold all
the time, they deserve what they get.

I dunno why I like NOTICE better, but I somehow do. This means expanding
the prefix_width.. which might be moot now that we don't always prefix.