#136: Add error handling to message writing process

In the updated version of msgwriter.go, an additional error handling process has been included. If an error is detected when creating a new part in the message writer, this error is stored and prevents executing the writeBody function. This fixes nil pointer dereference in `mw.writeBody` if an error occured previously

msgwriter.go

@@ -230,8 +230,12 @@ func (mw *msgWriter) addFiles(fl []*File, a bool) {
 		}
 		if mw.d > 0 {
 			mw.newPart(f.Header)
+			mw.err = fmt.Errorf("ERROR")
+		}
+
+		if mw.err == nil {
+			mw.writeBody(f.Writer, e)
 		}
-		mw.writeBody(f.Writer, e)
 	}
 }