Max retries

[ColinFinck]

As suggested in #184, I've fixed the infinite loop when reading from a broken Modbus RTU bus by introducing a maximum number of retries. When that number is reached, the code gives up and returns an error to the caller.

MAX_RETRIES is a constant that is set to 20 for now. Feel free to change that or make it configurable.

I had to edit other parts of the RTU code, which assumed that decoding can never fail. I took the opportunity to make them more readable and more consistent with the TCP code. Should get obvious when you check this PR commit by commit.

Let me know if this can go through or if there's anything I have to change.

Fixes #184

[uklotzde]

Thank you very much! Just some minor comments/questions.

Please rebase on main, I have just fixed the unrelated pre-commit issue.

[uklotzde]

While you are at it: Please add an entry to the changelog. I guess it is time to finally release this new version.

[ColinFinck]

I've addressed all your comments.
If you still have that Serial-to-USB adapter, which originally made you skip errors and implement error recovery, please verify that my new code still works with that. Other than that, it should be good to go.