Better error detection and notification

[mightybyte]

In cases where your markup has inconsistencies the error detection/notification is really bad. I've been bitten numerous times by these problems and they are pretty hard to track down. I don't have any concrete suggestions right now. Just wanted to start a dialog.

[mightybyte]

I looked into this a little more, and it seems like much of the problem is related to error handling with the "error" function. I fixed this in df-heist here:

Soostone@bbb129d#diff-a32ca51d16d0f098cf27e8e7d3c682f3L168

But there are still a bunch of places in the main library where error is used. The next major release of heist will have built-in error handling facilities, so from the df-heist standpoint I'll be able to deal with failure in a principled way. It would be really nice of df itself could transition to Either String functions so we can catch more error conditions in this way.