Update Tags.java

[limfriend]

add tag 'error_kind'

[coveralls]

Coverage decreased (-0.2%) to 76.009% when pulling abc408c on limfriend:patch-1 into d91d707 on opentracing:master.

[whiskeysierra]

https://static.javadoc.io/io.opentracing/opentracing-api/0.33.0/io/opentracing/log/Fields.html#ERROR_KIND

It's a log field, not a tag.

[whiskeysierra]

See also https://github.com/opentracing/specification/blob/master/semantic_conventions.md

[limfriend]

https://static.javadoc.io/io.opentracing/opentracing-api/0.33.0/io/opentracing/log/Fields.html#ERROR_KIND

It's a log field, not a tag.
'error.kind' is very important , we should be easy to search and stat it. it is difficulty to search when ''error.kind' is log field