drm/i915/selftest: Better error reporting from hangcheck selftest

There are many ways in which the hangcheck selftest can fail. Very few
of them actually printed an error message to say what happened. So,
fill in the missing messages.

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210727002348.97202-27-matthew.brost@intel.com