E3-1694: Remove parsing of iocsh Python exceptions
Provide iocsh stderr output in IocshProcessError instead of parsing it.
Currently the tests fail, because they expect FileNotFoundError but get IocshProcessError, so the question is if something else relies on the exception type. It doesn't seem to be the case at least for require tests.
Interestingly, it seems to fail only for Python 3.7, not exactly sure why...
Closes E3-1694