added new constants for error messages. still need to figure out the stack usage.
created a subroutine for printing a string on to the screen

But there is an error. we must find a way to integrate the stack into
this subroutine. and others. try to understand how stack works in
general though.
added the load stack pointer instruction

and an empty line for better appearance. Also, commented the code as one
should. make sure to comment as much as possible. good practice is to
comment each line almost right away even if it seems really obvious.
keep in mind, that it may be obvious to you but not to your teammates.
thanks.
corrected the default address for the stack pointer