`ErrorMessage.error_message` is a leftover from an older version of SSZ
that was able to encode unbounded lists. This is no longer the case -
all collection types now have a fixed upper bound on length.
In general, the `error_message`, just like the `graffitti` field, should
not be interpreted in any particular way except for debugging and vanity
- as such, using the same type, a `Bytes32`, seems reasonable.
An alternative would be `List[byte, 256]` which maybe could be
"reasonably backwards compatible" with whatever clients are are doing
now - depending on how they are dealing with this field type that no
longer exists in the SSZ spec :) It would however be the only place
where `List[uintN, N]` is used in the current spec.
As an exercise, this could be considered a security issue since it's
essentially unbounded and undefined behaviour.
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