Report conversion errors from ADC

[jannic]

Also provide an inherent read method so the ADC can be used without e-h 0.2

Closes: #740

[jannic]

Some doc tests still need to be updated.
Done.

[9names]

Changes look good.
Might be nice for the examples to do something about errors though - the warning comments are good but maybe we should be a better example to any would-be users?
It's not a blocking concern in any case.

[jannic]

Changes look good. Might be nice for the examples to do something about errors though - the warning comments are good but maybe we should be a better example to any would-be users? It's not a blocking concern in any case.

I wanted to add error handling, but I failed to come up with a good approach for such an easy example. Correct error handling depends on the use case, and the example doesn't have one. Sure, it could log an error message, but that wouldn't be much better than the comment.