validator: split validation issues from operation errors

[torkelrogstad]

Alternative to #55 and #54

Invalid MX messages are "okay" - in that they shoudln't crash our chain sync progress. We therefore separate them from operational errors, such as writing/reading to/from the database and other issues that IS in fact things that should take us down.

Some of these message are just fine to ignore, whereas some of them should result in blocl invalidation. This comes later!

[nchashch]

I don't like that the Ok variant of the Result can contain an error, that can be confusing.

55 is more idiomatic.

So I'll merge #55 and close this one.