Raise RuntimeError with better error messages

[sobolevn]

While working on #15776 I've noticed that some RuntimeError do not have enough metadata to understand what is going on.
CI: https://github.com/python/mypy/actions/runs/5700479199/job/15450345887

This is the traceback I got:

 _______________________ testNamedTupleNestedClassRecheck _______________________
[gw1] linux -- Python 3.10.12 /home/runner/work/mypy/mypy/.tox/py/bin/python
data: /home/runner/work/mypy/mypy/test-data/unit/fine-grained.test:10213:
/home/runner/work/mypy/mypy/.tox/py/lib/python3.10/site-packages/_pytest/runner.py:341: in from_call
    result: Optional[TResult] = func()
/home/runner/work/mypy/mypy/.tox/py/lib/python3.10/site-packages/_pytest/runner.py:262: in <lambda>
    lambda: ihook(item=item, **kwds), when=when, reraise=reraise
/home/runner/work/mypy/mypy/.tox/py/lib/python3.10/site-packages/pluggy/_hooks.py:433: in __call__
    return self._hookexec(self.name, self._hookimpls, kwargs, firstresult)
/home/runner/work/mypy/mypy/.tox/py/lib/python3.10/site-packages/pluggy/_manager.py:112: in _hookexec
    return self._inner_hookexec(hook_name, methods, kwargs, firstresult)
/home/runner/work/mypy/mypy/.tox/py/lib/python3.10/site-packages/_pytest/runner.py:177: in pytest_runtest_call
    raise e
/home/runner/work/mypy/mypy/.tox/py/lib/python3.10/site-packages/_pytest/runner.py:169: in pytest_runtest_call
    item.runtest()
/home/runner/work/mypy/mypy/mypy/test/data.py:320: in runtest
    suite.run_case(self)
/home/runner/work/mypy/mypy/mypy/test/testfinegrained.py:113: in run_case
    output, triggered = self.perform_step(
/home/runner/work/mypy/mypy/mypy/test/testfinegrained.py:217: in perform_step
    new_messages = self.run_check(server, sources)
/home/runner/work/mypy/mypy/mypy/test/testfinegrained.py:167: in run_check
    response = server.check(sources, export_types=True, is_tty=False, terminal_width=-1)
/home/runner/work/mypy/mypy/mypy/dmypy_server.py:416: in check
    messages = self.fine_grained_increment(sources)
/home/runner/work/mypy/mypy/mypy/dmypy_server.py:562: in fine_grained_increment
    messages = self.fine_grained_manager.update(changed, removed)
/home/runner/work/mypy/mypy/mypy/server/update.py:267: in update
    result = self.update_one(
/home/runner/work/mypy/mypy/mypy/server/update.py:369: in update_one
    result = self.update_module(next_id, next_path, next_id in removed_set, followed)
/home/runner/work/mypy/mypy/mypy/server/update.py:452: in update_module
    remaining += propagate_changes_using_dependencies(
/home/runner/work/mypy/mypy/mypy/server/update.py:881: in propagate_changes_using_dependencies
    triggered |= reprocess_nodes(manager, graph, id, nodes, deps, processed_targets)
/home/runner/work/mypy/mypy/mypy/server/update.py:1017: in reprocess_nodes
    merge_asts(file_node, old_symbols[name], file_node, new_symbols[name])
/home/runner/work/mypy/mypy/mypy/server/astmerge.py:139: in merge_asts
    node = replace_nodes_in_ast(new, replacement_map)
/home/runner/work/mypy/mypy/mypy/server/astmerge.py:191: in replace_nodes_in_ast
    node.accept(visitor)
/home/runner/work/mypy/mypy/mypy/nodes.py:377: in accept
    return visitor.visit_mypy_file(self)
/home/runner/work/mypy/mypy/mypy/server/astmerge.py:212: in visit_mypy_file
    super().visit_mypy_file(node)
/home/runner/work/mypy/mypy/mypy/traverser.py:114: in visit_mypy_file
    d.accept(self)
/home/runner/work/mypy/mypy/mypy/nodes.py:1141: in accept
    return visitor.visit_class_def(self)
/home/runner/work/mypy/mypy/mypy/server/astmerge.py:237: in visit_class_def
    self.process_synthetic_type_info(info)
/home/runner/work/mypy/mypy/mypy/server/astmerge.py:404: in process_synthetic_type_info
    node.node.accept(self)
/home/runner/work/mypy/mypy/mypy/nodes.py:206: in accept
    raise RuntimeError("Not implemented")
E   RuntimeError: Not implemented

This PR adds more context to error messages.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[sobolevn]

Going to merge, as this is quite minor.