Catch-all error can be quite misleading

[odunbar]

issue

This error message:

CalibrateEmulateSample.jl/src/Emulator.jl

Lines 40 to 51 in 101d3f6

	function throw_define_mlt()
	throw(ErrorException("Unknown MachineLearningTool defined, please use a known implementation"))
	end
	function build_models!(mlt, iopairs, input_structure_mats, output_structure_mats, mlt_kwargs...)
	throw_define_mlt()
	end
	function optimize_hyperparameters!(mlt)
	throw_define_mlt()
	end
	function predict(mlt, new_inputs; mlt_kwargs...)
	throw_define_mlt()
	end

designed to catch unknown or unimplemented machine learnign tools can be misleading. Primarily, if one gets the data-type incorrect for the inputs in say predict(mlt, inputs...) it throws the error implying that the mlt argument was incorrect!

Should be corrected to a more specific message, and not thrown in these cases to avoid confusion.