Fixed a -Werror=unused-result in OptionParser.cc
Fixed a -Werror=unused-result in OptionParser.cc

OptionParser::invokeGDB() calls system(...) to attach GDB to the
process, but does not check the return value, causing a unused-result
error on Ubuntu. It now checks the output and prints an error message
to stderr upon failure.

SMatsushi/RAMCloud,utah-scs/RAMCloud,anirajk/RAMCloud,anirajk/RAMCloud,jblomer/ramcloud,jblomer/ramcloud,jblomer/ramcloud,jblomer/ramcloud,jblomer/ramcloud,utah-scs/RAMCloud,SMatsushi/RAMCloud,anirajk/RAMCloud,anirajk/RAMCloud,SMatsushi/RAMCloud,utah-scs/RAMCloud,anirajk/RAMCloud,SMatsushi/RAMCloud,utah-scs/RAMCloud,utah-scs/RAMCloud,SMatsushi/RAMCloud,anirajk/RAMCloud,jblomer/ramcloud,SMatsushi/RAMCloud,utah-scs/RAMCloud