feat(go): use GenkitError instead of regular error

[sahdev77]

Addition of new Genkit error in go:

• Handling Error using Genkit Error.
• Removal of HTTPError from misc.
  Resolving : 2476
  Checklist (if applicable):
• PR title is following https://www.conventionalcommits.org/en/v1.0.0/
• Tested (manually, unit tested, etc.)
• Docs updated (updated docs or a docs bug required)

[apascal07]

Can you modify a sample to return errors in various places and verify in the Dev UI that the errors look correct and have a useful stack? Conversely, the stack should not appear when calling the prod server, only the reflection server.

[apascal07]

There are a lot of exported constants here that aren't qualified, it might make sense to put this in a error directory and package rather than pollute the core namespace.

[sahdev77]

so are you suggesting to create a error package all together ?
As of now it is in sync with the structure that we had in Js and py but go behaves differently so we can think of keeping it as a separate package itself(genkitErr)

[apascal07]

JS and Python allow importing things piecemeal instead of the entire package like in Go so it's more important for things to be encapsulated well.