Clarify when error tag is used

[pavolloffay]

The specification defines that Tags.ERROR should be used if there is any error. This definition is vague. Should it be added for response codes > 400 or when an exception is thrown which results in 500?

Instrumentations usually add error tag in exception states e.g. when the modeled operation failed or an exception is thrown.

true if and only if the application considers the operation represented by the Span to have failed

It is also a best practice to add the exception to logs see https://github.com/opentracing/specification/blob/master/semantic_conventions.md#captured-errors