error constructors

[corybrunson]

See this chapter for an intro to error constructors.

I see several opportunities for these, possibly including in the generated code (where it might still save development time if shared by non-generated code):

• unrecognized persistent homology class: pairs_min(), pairs_max()
• inadequate persistence data: get_pers_max_frac(), get_pers_min_mult()
• list-column requirement: check_pd_list(), get_pairs_max(), prep.step_pd_*() and other steps that take dataset columns as input, get_blur_range() and other dials for dataset-valued variables