Save 868 bytes of RAM by shortening debug output - we were running out of memory!

Arduino Uno (2KB RAM) crashes without these changes.

date + bss MUST be (significantly?) smaller than RAM size - see
http://playground.arduino.cc/Main/CorruptArrayVariablesAndMemory

avr-size .build/uno/firmware.elf
before:
   text	   data	    bss	    dec	    hex	filename
  16240	   1536	    309	  18119	   46c7	.build/uno/firmware.elf

after:
   text	   data	    bss	    dec	    hex	filename
  15454	    668	    309	  16431	   402f	.build/uno/firmware.elf
Convert all error code return values to errv_t (unsigned char)

This should save 1 byte of stack space for each function call.