[Go] Improve errors throughout by adopting GenkitError and UserFacingError

[apascal07]

Describe the bug
Right now, we just return errors (sometimes HTTPError) throughout Genkit Go which lacks the expected status codes, no breakdown of user vs. internal errors (INVALID_ARGUMENT vs INTERNAL, etc), many cryptic errors coming straight from the plugin APIs. We also do not capture or propagate any stack trace because Go doesn't do this by default but this is very helpful for developers for libraries like this.