Improved message on config file read error

Previously, "Error: could not find config file ???" was
printed when a config file could not be opened. This also includes when
the file had the wrong read permissions. Thus, an incorrect error
description was given (since the file actually existed).

This commit corrects this behaviour. This is done by introducing the new
error message "Error: could not read config file ???". This message is
now presented when the config file exists, but can't be read. The
original message is only presented when the config file actually does
not exist.

Additional tests are committed that ensures correct behaviour.

This commit fixes issue #61.