mpsse: Display libusb error names instead of numbers

Added calls to libusb_error_name() where applicable in order to easier
understand the error messages.

Change-Id: I3fe3d4b5624ae0de37c36e54a371eba5535ccaa1
Signed-off-by: Joakim Gebart <joakim.gebart@eistec.se>
Reviewed-on: http://openocd.zylin.com/2289
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>