From《Identification cards － Integrated circuit cards - Part 4: Organization, security and commands for interchange》Page 56 :
In this group of commands, SW1-SW2 set to '6300' or '63CX' indicates that the verification failed, 'X' > '0'
encodes the number of further allowed retries. 

according to above, we can return SC_ERROR_PIN_CODE_INCORRECT when return 0x6300 .
We didn't find the exact error message which define verification failed, so we used the "SC_ERROR_PIN_CODE_INCORRECT" which most close to the current situation. 
BTW, how about add a new error message for example "SC_ERROR_VERIFICATION_FAILED"? That will be more  clear.