Changes the implementation of action execution to deal with the
returned associated errors.
The only sensible constraint on those errors that I could think of for
now is `Debug`, meaning that errors are now persisted as debug
messages.
This is not as nice to work with for a future implementation of
retryable actions as the equivalent in Haskell, but maybe an idea
shows up underway. The main issue is that most of the common error
types will not be implementing Serde traits, so serialization to/from
the same error type is difficult.
Adding an implementation constraint for JSON serialisation on error
types (i.e. `S::Error: Serialize + Deserialize`) would probably cause
headaches for users, especially if they are trying to use an
out-of-the-box error type or an error type wrapping foreign errors.
Det ska'kke være lett ...
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