Fix race condition in add_outbound_files() causing "Unable to open FLO file".

The directory list is catpured *before* the lock file is "taken", so it can
happen that another process (e.g. instance of binkit) has dealt with one or
more of the files by the time we take the lock and attempt to open the file.

Also, updated error log messages a bit.
3 jobs for master in 19 minutes and 31 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #998
build-sbbs

00:16:52

passed #999
build-sexpots

00:00:14

passed #1000
build-syncterm

00:02:25