ADC should report conversion errors

[jannic]

The ADC peripheral can report conversion errors. The datasheet explains:

Conversion errors produce undefined results, and the corresponding sample should be discarded. They indicate that
the comparison of one or more bits failed to complete in the time allowed. Normally this is caused by comparator
metastability, i.e. the closer to the comparator threshold the input signal is, the longer it will take to make a decision.
The high gain of the comparator reduces the probability that no decision is made.

Currently, we just ignore the error flag, and report the conversion result as if it was valid.