Some error messages, for example in exceptions thrown in gr_simple_flowgraph, could be more helpful if you give the name and unique_id of the affected block.

For example, gr_simple_flowgraph_detail::check_contiguity() throws std::runtime_error("insufficient ports"). Which block, which port was involved?

The exceptions get shown in python like this:
File "... gnuradio_swig_py_runtime.py", line 1477, in runtime_run_unlocked
return _gnuradio_swig_py_runtime.runtime_run_unlocked(*args)
RuntimeError: insufficient ports