Stop asking users to report peer errors, fix a common peer error

[teor2345]

Motivation

Zebra asks users to report peer errors, but there's nothing that Zebra developers can do to fix most of those errors.
This PR fixes one of the errors we can fix. #2107 should fix the rest.

This is unexpected work in sprint 22.

Solution

• Stop treating inv with mixed item types as a connection error. Instead, ignore any unexpected item types.
• Remove unused connection errors. We'll remove more errors in Security: Stop disconnecting from nodes that send unexpected messages, to prevent disconnection attacks, Credit: Equilibrium #2107.
• Stop asking users to create bug reports for peer errors.

Closes #3052.

Review

Anyone can review this low-priority bug fix.

Reviewer Checklist

• Code handles network messages correctly

This code needs to be refactored before we can unit test it. But it's tested by the zebrad acceptance tests.

Follow Up Work

• Security: Stop disconnecting from nodes that send unexpected messages, to prevent disconnection attacks, Credit: Equilibrium #2107

[oxarbitrage]

Looks good.