Debug flag for `map_over_subtree`

[jbusecke]

This is a common scenario for me:
I construct a tree from many different datasets, manipulate, and plot it.
Now let's say, something unexpected happened, that will throw an error during plotting of only one/a subset of datasets (e.g. the dimensions of one of the datasets was larger than expected hence the plot fails).

If there was a way to get the exact path of the failure

dt.map_over_subtree(plot_func, debug=True) --> "An error occured while trying to map function over `sub/tree/path/to/dataset`"

That would speed up finding issues with inhomogeneous data.

[TomNicholas]

👍 We should just improve the error handling so that the stack trace gives context as to which dataset the error was raised from.

[jbusecke]

Even better! Less complex.