Add transaction.name to errors

[devcorpio]

Summary

A while ago, there was a request for APM agents to add the transaction.name to errors, the goal of that was:

"the UI could show a distribution of transaction groups for an error group which will help the user understand if it's a general problem, or specific to a single transaction group."

To do

The RUM agent should also add transaction.name to errors

Please, see the issue below for more details:

elastic/apm#531