queue sys.exit in testing.

[Carreau]

#139 pinned ipykernel,

A small change means that all executions requests are now cancelled after an error; this means in particular that in testing the exist() requests we are sending get cancelled when we test for exceptions.

A likely workaround is to queue requests and wait for the results of previous action before submitting new ones.

It is not critical right now, but will probably be needed in the future.