Print user friendly error message for serf error codes in serf_get utility.
Print user friendly error message for serf error codes in serf_get utility.

* test/serf_get.c
  (main): Use serf_error_string().

lazysoez/serf,x-clone/serf,alex1818/serf,stoneCC/serf,lazysoez/serf,yesoce/serf,zhengcq315/serf,pengdu/serf,yesoce/serf,zhengcq315/serf,google-code-export/serf,alex1818/serf,x-clone/serf,SirJson/serf,pengdu/serf,pengdu/serf,zhengcq315/serf,yesoce/serf,lazysoez/serf,google-code-export/serf,x-clone/serf,KyleSanderson/serf,pengdu/serf,lazysoez/serf,KyleSanderson/serf,KyleSanderson/serf,soufianeEL/serf,soufianeEL/serf,stoneCC/serf,stoneCC/serf,zhupan/serf,zhupan/serf,SirJson/serf,soufianeEL/serf,google-code-export/serf,SirJson/serf,alex1818/serf,soufianeEL/serf,x-clone/serf,KyleSanderson/serf,zhengcq315/serf,google-code-export/serf,zhupan/serf,SirJson/serf,yesoce/serf,alex1818/serf,stoneCC/serf,zhupan/serf