Gracefully handle missing mbed.htm on an STLink MSD volume.
Gracefully handle missing mbed.htm on an STLink MSD volume.

mesheven/pyOCD,pyocd/pyOCD,mbedmicro/pyOCD,flit/pyOCD,mesheven/pyOCD,mbedmicro/pyOCD,mbedmicro/pyOCD,mesheven/pyOCD,pyocd/pyOCD,flit/pyOCD