siso: print error if siso binary doesn't exist in third_party/siso

If build/config/siso/.sisoenv exists, but third_party/siso/cipd/siso
doesn't exist, it prints error

depot_tools/siso.py: Could not find .sisoenv under build/config/siso of the current project. Did you run gclient sync?

But it should print error as siso binary not in third_party/siso
instead.

Change-Id: I6a6a920b0e6d877551f0fa7c650ab7c8165180bf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5802659
Auto-Submit: Fumitoshi Ukai <ukai@google.com>
Commit-Queue: Fumitoshi Ukai <ukai@google.com>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>

siso.py

@@ -114,6 +114,11 @@ def main(args):
             if siso_path and os.path.isfile(siso_path):
                 checkOutdir(args[1:])
                 return subprocess.call([siso_path] + args[1:], env=env)
+        print(
+            'depot_tools/siso.py: Could not find siso in third_party/siso '
+            'of the current project. Did you run gclient sync?',
+            file=sys.stderr)
+        return 1
     if siso_override_path:
         return subprocess.call([siso_override_path] + args[1:])