Prevent `emitError` from causing a runtime error which prevents `changeVisibilityTimeout` from being called.

[glennflanagan]

Description

If handleMessage rejects with undefined or null then emitError will receive the same and will cause runtime error of TypeError: Cannot read property 'name' of undefined and will prevent changeVisibilityTimeout from being called.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)

  Checklist:

• [x] My code follows the code style of this project.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[nicholasgriffintn]

I think really, handleMessage should just always reject with a proper error message, we definitely don't want to change our types here because of that, internally, we are always expecting an error to be an error, and would just external functions do the same.

[github-actions[bot]]

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