Trapperkeeper Config Service should log a warning when a file in the config directory is not readable.

Using puppet-server as an example, when a config file in
/etc/puppetserver/conf.d is not readable by the user who owns the java process
under which puppet-server is run the Trapperkeeper Config Service moves on to
the next config file without warning that it encountered a file which could not
be read.

This leads to a somewhat difficult to troubleshoot problem later on when some
other trapperkeeper service attempts to use a value provided by that config file
but can't find it.

An even more difficult to troubleshoot situation would be one in which that
config value falls back to an acceptable default instead of failing when it
can't find the one provided by the user config file. This is more difficult
because rather than the entire service stack failing loudly there may simply be
subtle behavioral differences that go undetected until something bad happens,
possibly days/weeks/months after the config change.