Give a more descriptive error messsage when someone tries to write an…

… object of an unsupported class.

lib/row.rb

@@ -148,7 +148,7 @@ def write(col, label, style)
     when ExcelFormula
       @cells << FormulaCell.new(self, col, style_index, label)
     else
-      raise label.class.name
+      raise "You are trying to write an object of class #{label.class.name} to a spreadsheet. Please convert this to a supported class such as String."
     end
   end