coerceVariableValues might log sensitive data in case of error

The prefix in `prefix + '; ' + error.message,` ([execution/values.js](https://github.com/graphql/graphql-js/blob/master/src/execution/values.js):141) logs the invalid input when a call to `coerceVariableValues` fails.

Since this might commonly fail, especially when invalid user input accidentialy reaches the API, according log messages can contain sensitive (user) data, if transmitted in the request:

```While handling a GraphQL request the following error occurred: Variable "$myInput" got invalid value { abc: "xyz", address: { city: "SecretCity", country: "SecretCountry", state: "SecretState", streetName: "An Interesting Street", streetNumber: "42", zipCode: "12345" }, ... }; Field "abc" is not defined by type MyInput.```

Please clarify how we could solve this problem (we can provide a PR too), or enlighten me if we happen to use the library in a wrong way.