Creating a Custom ToolPart(Editor Panel) and VALIDATION for a Custom Web Part

https://www.nothingbutsharepoint.com/sites/devwiki/articles/Pages/SharePoint-Creating-a-Custom-ToolPart-for-a-Custom-Web-Part.aspx
http://waelmohamed.wordpress.com/2010/03/14/create-custom-web-part-properties-using-custom-tool-parts-with-a-complete-example-code/

Validation code for editor panel Properties:

using Microsoft.SharePoint.WebPartPages;

private string headline = String.Empty; 

public string Headline 

{ 

get {return headline;} 

set 

{ 

If(String.IsNullOrEmpty(value) 

{ 

throw new WebPartPageUserException (“Please enter Headline”); 

} 

else 

{

headline = value;  

}

}

}

http://zootfroot.blogspot.in/2010/09/develop-custom-editable-visual-web-part.html