DRV-567: query error output should be consistent with fauna web shell

[vadimLF]

Based on #83 and https://faunadb.atlassian.net/browse/DRV-567.
When error occurs in web-shell user observes the following error output:

Error: [
  {
    "position": [],
    "code": "call error",
    "description": "Calling the function resulted in an error.",
    "cause": [
      {
        "position": ["expr"],
        "code": "invalid ref",
        "description": "Ref refers to undefined index 'user_ref_by_username'"
      }
    ]
  }
]

the same error output in go driver looks like this:
Response error 400. Errors: [](call error): Calling the function resulted in an error.

which means that go driver does not display cause field.

[erickpintor]

LGTM

[vadimLF]

Let's put this PR on hold since we want to consolidate errors for all drivers.