title: Material-UI Datepicker can't display error/required messages
body: <p>I am trying to show a validation error message on Date picker like how we do for TextField using errorText and errorStyle props but the validation error message is not working for DatePicker.</p>
  
  <p>I am using material-ui version "^0.18.7".</p>
  
  <p>Please find full functionality below. I am not sure exactly how to implement this. I also understand that the errorText and errorStyle is not a valid prop to DatePicker. Thought it will work for datepicker but it is not. Is there a way to achieve this.</p>
  
  <pre><code>import DatePicker from 'material-ui/DatePicker';
  
  constructor(props){
     super(props)
     const dateYesterday = new Date();
     dateYesterday.setDate(dateYesterday.getDate() - 1);
     this.state={
        dob: null,
        dobError: '',
        dateYesterday: dateYesterday
     }
     this.changeDateOfBirth = this.changeDateOfBirth.bind(this)
  }
  
  changeDateOfBirth = (evt, date) =&gt; {
  if(date == null || date == undefined){
    this.setState({
      dobError: 'Please select your date of birth'
    });
  }else{
    this.setState({
      dobError: '',
      dob: date
    });
  }
  }
  const errorStyle = {
      display: 'table'
  }
  
  &lt;DatePicker 
      dialogContainerStyle={{position: 'absolute'}} 
      errorText={this.state.dobError} 
      errorStyle={errorStyle} 
      inputStyle={myTheme.inputStyleText} 
      underlineStyle={myTheme.underlineStyle} 
      underlineFocusStyle={myTheme.underlineStyle} 
      floatingLabelStyle={myTheme.floatingLabelStyle} 
      floatingLabelText="Date Of Birth" 
      hintText="Select Date Of Birth" 
      container="inline" 
      locale="en-US" 
      firstDayOfWeek={0} 
      autoOk={true} 
      value={this.state.dob} 
      onChange={this.changeDateOfBirth} 
      defaultDate={this.state.dateYesterday}
   &gt;
   &lt;/DatePicker&gt;
  </code></pre>
  
  <p>Please suggest.</p>