[Elixir] Fixes Typespec for multiple return definitions

[mrmstn]

This PR fixes the wrong typespec for mutiple return types. (Closes #3958)

It does first building and generating a list for every return type defined. Those are saved in a HashList to ensures that every defintion will only be present once.
After that, the list will be added to the existing StringBuffer and adding the {:error, Tesla.Env.t} at the end of the typespec.

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  @wing328 Since you're the only one I know here, could you do me the favor and review my changes :)?