Refactor pyDAPAccess transfer response checking.
Refactor pyDAPAccess transfer response checking.

- Factor out transfer response checking into a new method.
- Add a description to the exception for unexpected ACK values.

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