span_fn should add tag error=true for functions returning Result when is_err is true

[toffaletti]

It would be nice to automatically get error tags, which jaeger and other frontends have special ui treatment for.

[evanrittenhouse]

You can assign this to me, it'd be nice to have for some stuff we're working on. I'll have to figure out how to handle the case where an error tag is already added. For now I think it's fine to just exclude it if the tag has it