Improve error message when wrong number of hyperparameters

[orasis]

When using 2 hyper parameters when 3 is expected, the current error is "Request failed with response 500: None
list index out of range"

This took me about 20 minutes to hunt down. A more explicit error message stating that the wrong number of hyper parameters was provided would be better.

[suntzu86]

Sorry I've been gone for so long! See: #448 for details.

I'll add this to my upcoming to-do list. Yup, I agree, the I/O bounds/type/etc checking is pretty weak. You can pass in inf and NaN right now too in place of numeric parameters and watch things break :/

num hyperparams in particular is tougher b/c the precise number is dependent on the covariance kernel. But I'll make report an error properly.

edit: @orasis ^^