Add a method to reset the ok flag and clear the error message

[stfufane]

Hi,

I don't know if you might be interested or not by this pull request, but I'm making a wrapper around the library with Deno, something similar to what was made here for Node JS : https://github.com/justinlatimer/node-midi

And while trying to handle errors, I noticed two things :

• The msg member of the device struct was set from a reference to an exception, and was then incorrect when the exception got out of scope (dangling pointer)
• Once an error was met, the ok flag was set to false forever

It's kinda specific to that type of usage though, because I call the library with FFI bindings and then read the methods' results, so it's probably not required for a "classic" usage linking to a native C/C++ application.

Anyway, I'm open to your opinion on that :)

Cheers