Don't confuse an unreadable pidfile directory as meaning there is no pidfile.

The pidfile directory is by default not readable by non-members of the server group. This previously confused our code into thinking the pidfile simply didn't exist, meaning the server was not running, even if it actually was.

Also make relative-path local updates actually work (I introduced this a couple commits ago but apparently failed to test it properly).

Fixes #51. Should also improve issue #49 by at least making the error clearer.