This fixes some errors that have been appearing in our CI from time to
time. All transformations are implemented as generators, but they
haven't been explicitly aborted when an exception has been raised.
Instead, they only got closed when they got garbage collected, which
could happen at an unspecified later time, potentially leading to a
corruption of global state, which could have been modified after the
exception was handled.

Note that this implementation doesn't propagate the original exception
into the argument transformations, and doesn't allow them to handle the
error either. Such an extension would be possible, but throwing an
exception into a generator mutates the exception object, clobbering
the nice traceback that we would usually carry. One can work around
those issues, but it feels really hacky and we don't need it right now
anyway, so I figured we'll be better off with the simple thing for the
time being.