[TODO] Add clearer error messaging to package

[edunford]

Problem

Issues like the one encountered in #32, where simple input errors are made but no errors are thrown notifying the user of incorrect inputs and how to rectify them, is a source of confusion for downstream users.

Proposal

Add a more comprehensive checks regime to the core instantiation function synthetic_control() and better error handling throughout the package to adequately inform users when usage errors are being made.

The checks regime should ensure that:

• The intervention unit is actually present in the supplied unit field.
• The intervention time point is actually present in the supplied time field.
• The supplied time field is either an int or Date object (but not a string).
• There is no unit with complete missingness for any one of the predictor variables (this check would need to live in the generate_predictor() predictor function.

If you see this and this issue-task is still open, please supply other checks you think might be useful (i.e. are there any user-error friction points that you've run into that a useful error and check would have spotted quickly if in place?).