Better error reporting for incomplete values

* Currently we descend into expressions to find the parts that are incomplete and reporting these. This is already a huge step over saying _something in this resource is incomplete, figure it out_
* However since we evaluate locals eagerly we cannot descend into expressions assigned to local variables
* Separately, if a variable `foo` is incomplete and used in 3 places in an expression we report it 3 times

Figure out a way to make this better.

Also some parts of the code do a poor job with diags capture. Maybe simplify and standardize this across the board.