Depth limit validator key error if fragment undefined

[ahmedaljawahiry]

Current behaviour:
When using the depth_limit_validator (docs), an invalid query can lead to a KeyError being thrown (from here), which usually results in the API returning a 500 server error.

To reproduce:

• Add the depth_limit_validator to the validation_rules in your GraphQLView.
• Execute a query with an undefined fragment, i.e. something like:

query {
  validQuery {
    validField
    ...undefinedFragment
  }
}

Expected behaviour:
A standard validation error response, like:

{
  "errors": [
    {
      "message": "Unknown fragment 'undefinedFragment'.",
      "locations": [
        {
          "line": x,
          "column": y
        }
      ]
    }
  ]
}

Since this is something that's caught by the standard validators, depth_limit_validator should probably just skip such errors. Or, do whatever the other validators do when they hit an "unrelated" error.

Environment:

• Version: 3.3
• Platform: n/a