doc: finish reporting the ReadableConfig error

TODO.md

@@ -1,5 +1,4 @@
 # To-do
-- Report that "ReadableConfig" error, which was super unhelpful and very annoying when you don't know why it's bugging.
 - Find a better notation for the error-handling in read_config_template() – although the custom error message is desirable, it's very bad
 to have to implement it using such a ridiculously long line...
 - Look at clap feature to allow deriving an arg-group from an enum to vastly simplify the structs required for parsing