EuiFormRow complains about duplicate keys when provided with multiple errors that are nodes

[cjcenizal]

This was introduced by #685. EuiFormRow currently uses the error value as a key to discern between multiple errors:

    if (error && isInvalid) {
      const errorTexts = Array.isArray(error) ? error : [error];
      optionalErrors = errorTexts.map((error, i) => (
        <EuiFormErrorText key={error} id={`${id}-error-${i}`} className="euiFormRow__text">
          {error}
        </EuiFormErrorText>
      ));
    }

If the errors are nodes this will result in the following console warning:

warning.js:33 Warning: Encountered two children with the same key, `[object Object]`. Keys should be unique so that components maintain their identity across updates. Non-unique keys may cause children to be duplicated and/or omitted — the behavior is unsupported and could change in a future version.