Document how to customize error handlers

[kaylareopelle]

It seems like we expect users to be able to customize the OpenTelemetry error handler, but I am having trouble finding documentation (on OTel docs, within code, or RubyDoc.info) that explains how to do this. It seems like it's possible due to the attr_writer for :error_handler within the SDK configurator.

If we want users of the SDK to have the ability to customize the error handler, we should have easy to find instructions that describe how to do this.

If this documentation exists and I somehow missed it, please add links as comments on this issue.

[github-actions]

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