Create TOML::Error as a base class for all errors

[rsim]

Currently when validating user entered TOML and presenting the error messages in the user interface we need to

rescue TOML::ParseError, TOML::ValueOverwriteError => e

It would be better if there would be a base class TOML::Error from which TOML::ParseError and TOML::ValueOverwriteError (and any other TOML errors) would inherit and then simpler

rescue TOML::Error => e

could be used to rescue any invalid TOML errors.