Fix error-context for repeated fields
Currently errors for repeated fields are really hard to understand, e.g.:
[error] (Test / runMain) fs2.interop.scodec.CodecError: message/2/message/1/message/5/message/5/double/5/int32: cannot acquire 8 bits from a vector that contains 0 bits
This for repeating field 5; you can see that the later pieces don't make sense - primitive double followed by an int.