Handle errors in constructors

[jtobin]

A lot of Maybe values are examined a number of times, unnecessarily. It would usually suffice to examine stuff that we potentially could throw on, e.g. usually pointCursor, contracts, etc. in the constructor itself, rather than in the render method and elsewhere.

[jtobin]

Going to close this -- I think it's still likely to be smart practice, but is not the whole error-handling story, and is a bit too vague as-is.