handle pipe error

found with clang scan-build
ipc.c:101:5: warning: Function call argument is an uninitialized value
    pipe_sendCommand(pipeinfo->out, command);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
do not mix TokenError and BankIDError

found with clang:
bankid.c:245:25: warning: implicit conversion from enumeration type 'TokenError'
 to different enumeration type 'BankIDError' [-Wenum-conversion]
    BankIDError error = backend_storeCertificates(p7data, length, hostname);
                ~~~~~   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Document and simplify the IPC code