Follow up for #45 - separate Message values to a separate constant class

[wolframhaussig]

As part of #45, we implemented a solution for handling native errors. To make this easier, we provided the name of the msg-value in calls to SendMessage. This mapping from the value to the constant name causes issues as the same value may exist in multiple constants which have different meanings. Therefore, we should separate the constants that represent msg values into a separate class.