fix a comment after renaming command
fix and add some comments and debug msgs
change int to errv_t as done already for other fcts

Saves another 32 bytes:

Before:
avr-size .build/*/firmware.elf
   text    data     bss     dec     hex filename
  18596     698     273   19567    4c6f .build/leonardo/firmware.elf

After:
avr-size .build/*/firmware.elf
   text    data     bss     dec     hex filename
  18564     698     273   19535    4c4f .build/leonardo/firmware.elf
removing code duplication

Now it is possible to abort pouring after a BOTTLE_EMPTY error using the
hardware abort button at at ABORT_BTN_PIN.

Also saves 408 bytes.

Before:
avr-size .build/*/firmware.elf
   text	   data	    bss	    dec	    hex	filename
  18564	    698	    273	  19535	   4c4f	.build/leonardo/firmware.elf

After:
avr-size .build/*/firmware.elf
   text	   data	    bss	    dec	    hex	filename
  18222	    632	    273	  19127	   4ab7	.build/leonardo/firmware.el
introducing a hardware RESUME button

If pressed (i.e. connected to GND) after a BOTTLE_EMPTY error,
procedure continues (i.e. RESUME).