enhanced commit pipelines (fabric and orion)

[adecaro]

This PR introduces the following to the commit pipelines of fabric and orion:

• introduce ErrDiscardTX to explicitly signal that a transaction needs to be discarded even though it is valid for the backend.
• introduce validation message to be stored in the vault.

[alexandrosfilios]

the cases for Busy and HasDependencies implement the same logic. We could group them. Also we could remove one nesting by doing

for _, id := range deps {
...
}
if len(deps) > 0 {
return nil
}

But this is more of personal taste

[alexandrosfilios]

LGTM