Running multiple cells should stop on error

[rchiodo]

Came from a usability study

Perhaps with a flag to override

[ronglums]

By default run stops on the first error. Add a message "n cells are skipped due to the previous error. Use setting datascience.StopOnError to change the behavior." in the output if there're cells canceled due to error in the previous cell. The cells following the error cell will be removed in the interactive window.

[rchiodo]

This is likely going to require changes to how we submit as all of the cells have already been submitted to jupyter. Although the pendingCellSubscriptions could be used on an error to cancel all (but Jupyter might get to them first).

[rchiodo]

Preliminary bits in branch
rchiodo/stop_on_error