The previous logic that was intended to find the first non-empty
filename that represented a valid file location was wrong. Replaced with
a simpler filter-based version.
Additionally, if the user provides a specific configuration filename, we
no longer fall back to the defaults if it is not found. Instead we just
err out and inform them that the file the specified was not found.
When we can't find a config file, log all the locations we looked and
don't create a new file in $HOME/.ptkrc.
There is currently an intermittent bug in the config loading logic that
is leading to the default config file in $HOME/.ptkrc not being found
and then being overwritten with the default config. This is step one in
fixing it.
