Fix `outcome` definition in `error-tracking.md`

[gregkalapos]

In error-tracking.md the definition of how outcome is set contradicted what we define in tracing-transactions.md and in tracing-spans.md, especially the following:

Tracking an error that's related to a transaction does not impact its outcome.

This PR changes error-tracking.md to make it match the outcome definition from other parts of the spec (see links above). I think most agents already do this according to tracing-transactions.md and tracing-spans.md - so I expect we won't need to change any implementation in a specific agent.

If that's not the case, please raise it and let's discuss.

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