Expose the ST-Link status code on the raised exception

Context

I embed pyOCD as a library in an application that turns connection failures into actionable messages for end users, so I need to tell ST-Link failures apart programmatically.

Current behaviour

STLink._check_status() reads the numeric firmware status, formats it into the message via Status.get_error_message(), and does not carry the number any further:

status, = struct.unpack('<H', response)
if status != Status.JTAG_OK:
    error_message = Status.get_error_message(status)
    if status in self._ERROR_CLASSES:
        raise self._ERROR_CLASSES[status](error_message)
    else:
        raise exceptions.ProbeError(error_message)

The split in _ERROR_CLASSES reads as deliberate to me: the statuses mapped there are the SWD_* transfer errors, which have transport-agnostic equivalents in pyOCD's exception hierarchy, while the ST-Link firmware statuses (JTAG_*, BAD_AP, SWV_NOT_AVAILABLE) have no generic counterpart. I'm not suggesting that should change.

The consequence for a caller is that for those vendor-specific statuses the number is the only thing that distinguishes them, and it is currently present only inside the message text. These four all arrive as a bare ProbeError, but need different remedies:

Today we recover the number with a regex over the message. That works, but the message is composed rather than fixed in at least one path — TransferFaultError renders as Memory transfer fault (STLink error (17): AP fault) — so it seems a fragile thing to depend on.

Reproduction (no hardware)

from pyocd.probe.stlink.stlink import STLink
from pyocd.probe.stlink.constants import Status
from pyocd.core import exceptions

for status in (0x04, 0x0B, 0x0E, 0x41):
    cls = STLink._ERROR_CLASSES.get(status, exceptions.ProbeError)
    print(cls(Status.get_error_message(status)))

Checked on 0.45.1; stlink/constants.py and core/exceptions.py are unchanged since 0.37.0.

Suggestion

Carry the status on the exception, the way pyOCD already carries structured error data elsewhere (TransferFaultError.fault_address, FlashFailure.result_code): an optional status keyword on ProbeError, defaulting to None, passed from _check_status(). That leaves the message format and _ERROR_CLASSES untouched and is a no-op for callers that ignore it.

Would you take a PR against develop for this? Happy to hang it off a different exception class if ProbeError isn't the right place.

Start with STLink._check_status() in pyocd/probe/stlink/stlink.py and the ProbeError definition in pyocd/core/exceptions.py. Review how TransferFaultError.fault_address and FlashFailure.result_code carry structured data, then trace the existing ST-Link exception paths. Done means callers can read the numeric status while the current exception classes and message format remain unchanged.