Playtime: Possible error "different frame counts" on arrangement loop boundaries

[helgoboss]

Sentry: 5462043a02cb49c99826fc5336ee26c7

[helgoboss]

This revealed a bug in how Playtime handles smooth seek and playback at loop boundaries. I've fixed that, resulting in smooth playback (except for the MIDI reset which seems to be done by REAPER). I couldn't reproduce that particular error message, so I left it there and added some details to the message, in case it pops up later again.