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(Somehow I managed to forget to do this in commit 214d9038.)

This brings the program into full compliance with the principle that
the print statements to stdout or stderr should only happen in main().
The rest of the code now behaves like a library: it raises exceptions
to indicate errors and calls callbacks to indicate status checkpoints.

And, although you'd only know it from scrutinizing the diff carefully,
this change also fixes a Boolean grouping error in the conditional in
apply_meme() that governs the printing of the delay notification.

This section is no longer needed since commit 185c71f6.

And do some reformatting that this makes possible.

This simplifies the code a lot, even though it doesn't reduce the
size.  It's also preparation for handling multi-line text (issue #1).

Because this shifts some code around in a way that exacerbates the
already-insufficient commenting, improve the commenting too.