Tracked DoesNotExist: so you know where it came from

[oxinabox]

This was a discussion i had with Mike Innes about a year back.
The problem with DoesNotExist() is that you really have no idea where it came from.
Which in turn makes erroring on doing things with it problematic.

We could replace it with a macro (and probably rename it at the same time, #262).
If debugmode is enables tha macro could replace every instance with one that uses the same trick we use in for Thunks to capture the source location into a field.
Or for (much) higher performance cost, we could capture the stacktrace.

This would solve the problem that people sometimes have in Zygote where they are like "where did this nothing come from?"

[mzgubic]

Would this only introduce an overhead in debug mode, or always?

[oxinabox]

Only in debug mode