Ref T12857. This is a little murky, but this behavior seems clearly better.

When the disk is full, daemons may attempt to update their PID files and fail. When this happens, keep running rather than exiting.

• Made Filesystem::writeFile() throw an exception unconditionally to simulate a full disk.
• Ran bin/phd start.
• Saw daemons exit immediately.
• Applied patch, saw daemons stick around and do useful work instead.