Tilmann, the file-watch.tcl page actually does an ad_returnredirect at the end, so it'll incur another page hit, and hence, a reload of the procs you just asked it to watch. So you don't need to call apm_source out of order.
The APM can easily record when sourcing a file fails. This information could then be output on the APM index page showing the watches, via developer-support on every page, or in a third way, if we can think of something clever.
I'm rejecting this TIP, however. Discussion should've taken place in the "Development" forum first, so we could've had a thought through TIP.