#77 Change Bison error messages

[Craig E. Shea]

When Bison has a critical error that causes it not to be able to continue building the parser, it displays messages. However, these messages do not contain the words "warning" or "error". I suggest that prefixing messages generated by Bison with "warning" or "error" (according to their severity) so that, when this tool is used in environments such as Visual Studio, the error messages will appear in the Error List console instead of the generic Output window from the build process. In addition, the words "error" and "warning" provide developers information as to the severity of the message.