Added a message showing up when the program is not found

comsdk/communication.py

@@ -31,6 +31,11 @@ class Host(object):
         self.commands[cmd_name] = cmd
 
     def get_program_launch_path(self, prog_name):
+        if prog_name not in self.programs:
+            raise ValueError('Program "{prog_name}" is not recognized. '
+                             'Please add this program to "custom_programs" '
+                             'in the corresponding host in the config file '
+                             'if you want to use it.')
         path_to_prog = self.programs[prog_name]
         if path_to_prog is not None:
             return self.programs[prog_name] + '/' + prog_name