refactor: reduce duplication in the error-collector

[amychisholm03]

• Refactored _processUserErrors, _processTransactionExceptions, and _processTransactionErrors into a single function: _processErrors

Original Ticket:

• There's a lot of duplication here--for example, in the error-collector, _processUserErrors and _processTransactionExceptions are nearly identical, apart from the property they inspect. _processTransactionErrors behaves only slightly differently. Those three could be collapsed into one processor.
• First half of this Jira ticket (https://new-relic.atlassian.net/browse/NR-96722)

[jsumners-nr]

Looks good. I have some feedback on JSDoc formatting, and a couple items for consideration, but this is very close to ready.

[jsumners-nr]

We're really close. I believe that addressing these comments will finish it off.

[jsumners-nr]

Fantastic! Looks good.