Steady-state Error Definition
where E(s) is the error (and also the signal carried forward directly
  from the summing node), R(s) input and C(s) output.


The error is "demand" minus "output" and the output and the demand are desired to be equal hence, the "thing that does the math" is a subtractor: -



Picture from here.


  This definition has the slightly unsettling effect of yielding a
  negative error in the case that the output is above the reference
  signal, and vice versa.


If the controller, feedback network and plant (as shown above) are non-inverting, then "demand" minus "output" is absolutely correct in that the error produced drives the system towards closer accuracy.