Replace SubprocessError with FileNotFoundError

python/servo/command_base.py

@@ -537,7 +537,7 @@ def needs_gstreamer_env(self, target, env, uwp=False, features=[]):
         try:
             if check_gstreamer_lib():
                 return False
-        except subprocess.SubprocessError:
+        except (FileNotFoundError, WindowsError):
             # Some systems don't have pkg-config; we can't probe in this case
             # and must hope for the best
             return False