Add missing unit tests for custom errors in lisk-p2p - Closes#1043

[ishantiw]

Description

There are some missing test cases for custom error classes in lisk-p2p. Closes a task in #970

Review checklist

• The PR resolves #1043
• All new code is covered with unit tests
• All new code was formatted with Prettier
• Linting passes
• Tests pass
• Commit messages follow the commit guidelines
• Documentation has been added/updated

[ishantiw]

Addressed