Hi Chris,
you are trying to use stm32_mem.py to flash the discovery
board. It currently doesn't have the stm bootloader in
it's recognized devices, since to my knowledge no one
tested it with that.
You could either try to specify the "--product any" option
or simply use dfu-util instead of stm32_mem.
In latest master STLlink is the default (which should
also be more convenient, as you don't have to enter
bootloader mode manually first). Otherwise just select
DFU-UTIL from the flash mode drop down menu.