Detect unreachable `except` branches

Since except blocks are checked in order, and the first matching block is executed, it's possible to accidentally write a try-except that will never execute some branches. For example:

def bad_reader(path: str) -> bytes:
    if path == "bad":
        raise PermissionError

    with open(path, "rb") as f:
        return f.read()


def load_data(path: str) -> bytes:
    try:
        return bad_reader(path)
    except OSError:
        print("OSError caught")
    except PermissionError:
        # This branch will never be hit, since `OSError` will always catch
        # permission error
        print("PermissionError caught")

    return b""


load_data("bad")

Since PermissionError is a subclass of OSError, the except PermissionError case will never execute. I'd expect/hope that these kind of mistakes would be detectable with type information. I think adding checks for this would make sense under the --warn-unreachable config flag, so a new CLI option may not be needed.

The issue identifies the existing --warn-unreachable configuration as the intended entry point. Investigate how mypy checks exception types and define completion as reporting a later handler subsumed by an earlier handler, with coverage for the PermissionError/OSError ordering example.