Do not use AssertionError for control flow

When running in optimized mode, asserts are a noop (https://docs.python.org/3/reference/simple_stmts.html#the-assert-statement). We don't use optimized mode in the nightlies, but apparently it happens on the GRID (where the PYTHONOPTIMIZE environment variable is set), and since except AssertionError was used in one place for control flow, that lead to a different behaviour on the GRID.

tl;dr do not catch asserts in Python