[Trace API] Cleanup: Remove TraceError, TraceResult, and ExportError

[lalitb]

TraceError and TraceResult are defined in the OpenTelemetry API crate (source) and are used internally within the API implementation. Since the API itself should not return error objects, it would be preferable to either remove them from the API or move them to the SDK if they are needed there. The same consideration applies to ExportError.