[path] improve 'exists()' performance

call 'os.stat()' directly

gallery_dl/path.py

@@ -160,8 +160,12 @@ class PathFormat():
 
     def exists(self):
         """Return True if the file exists on disk"""
-        if self.extension and os.path.exists(self.realpath):
-            return self.check_file()
+        if self.extension:
+            try:
+                os.stat(self.realpath)  # raises OSError if file doesn't exist
+                return self.check_file()
+            except OSError:
+                pass
         return False
 
     def check_file(self):