More informative error messages

[TheEimer]

Both in newer and older plugin versions, the errors if something about the config is wrong are very nondescript (e.g. function output not being registered and that being the error when the problem really is that the budget variable for mf was undefined). Documenting a) which config items are necessary and b) explicitly throwing config related errors would make debugging easier.

[benjamc]

Error is catched now if budget variable is not set. Documentation is still a TODO