probe_flash: Introduce an error code for "other" probing errors

Previously probe_flash had the same return code for the case when
no chips were matched, and when some other error happened during
probing. However these are two different scenarios and it is useful
for the caller to distinguish between them.

In fact, the caller (libflashrom it is) wanted to distinguish
between "no chips found" and "some other probing error" from the
very beginning. libflashrom probe API documented returning special
error code for "other error".
However it was not possible to know when "other error" happened
because probe_flash never returned that back, it could only say
"no matched chips found".

This patch introduces -2 as "other error" code from probe_flash,
while -1 remains as "no chips found".
Both libflashrom probe APIs v1 and v2 are now handling "other error"
from probe_flash and return it to the API callers as was promised in
the documentation.

This also adds a unit test for error code propagation for "no chips
found" error.

Change-Id: I4a271550bea2b36c657c71ce6cb1927082663c3c
Signed-off-by: Anastasia Klimchuk <aklm@flashrom.org>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/88008
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Peter Marheine <pmarheine@chromium.org>

include/libflashrom.h

@@ -4,6 +4,8 @@
  * Copyright (C) 2010 Google Inc.
  * Copyright (C) 2012 secunet Security Networks AG
  * (Written by Nico Huber <nico.huber@secunet.com> for secunet)
+ * Copyright (C) 2025 Dmitry Zhadinets <dzhadinets@gmail.com>
+ * Copyright (C) 2025 Google LLC
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by