Improve error reporting for `OTLP::Exporter#encode`

[kaylareopelle]

Currently, we get error messages like, OpenTelemetry error: unexpected error in OTLP::Exporter#encode - Invalid argument for string field 'message' (given Class) when there's an error in the encoding step for traces/spans.

We've had multiple reports that this error message does not contain enough information to easily debug the problem (see: #1657 as one example).

Improve the error messages, or provide an easy option to get enhanced messages for users who aren't satisfied with the current messages.

For example, when I'm debugging locally, I often add the backtrace to the output to help locate the issue.