Extracting exception key to constant

Extracting converter exception keys to a constant allow users to see which keys are used in ResourceBundle, because Javadocs generate a page called "Constant Field Values".

When I refactor the code I see some wrong messages like `not_a_valid_interger` to a `Short`, `Byte` and `Long` converters. So I choose to use always `not_a_valid_number` as used by `byte`, `BigDecimal`, `Double` and `Float` uses. What you think about this change, devs?