Splits ConfigError reference into 'section key'

The previous ConfigError class had a 'Referenced Config' field and a
'fix isntructions' field. The 'referenced config' was supposed to let
the user know which key in config.txt gave them the error. This meant
that I had to type out '[Section] Key' every time. There was no reason
to do this.

Now, ConfigErrors has 3 fields: 'Section', 'Key', and 'Fix
Instructions'. Fix instructions is exacty like it was before, but now
'Referenced Config' has been split up into 'Section' and 'Key'. This
allows easier creation of ConfigError objects