Catch-all error can be quite misleading

## issue

This error message:

https://github.com/CliMA/CalibrateEmulateSample.jl/blob/101d3f6ece288e3d56f2fa51e829709f40f87e6b/src/Emulator.jl#L40-L51

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.