add offline error code r=sspitzer,sr=mscott 15865

mailnews/base/public/msgCore.h

@@ -129,6 +129,8 @@ NS_ERROR_GENERATE_FAILURE(NS_ERROR_MODULE_MAILNEWS, value)
 
 #define NS_MSG_FOLDER_EXISTS NS_MSG_GENERATE_FAILURE(19)
 
+#define NS_MSG_ERROR_OFFLINE NS_MSG_GENERATE_FAILURE(20)
+
 /* ducarroz: error codes for message compose are defined into compose\src\nsMsgComposeStringBundle.h.
              Message compose use the same error code space than other mailnews modules. To avoid any
              conflict, I reserve values between 12500 and 12999 for it.