[downloader:http] handle exceptions in 'validate'

This isn't strictly necessary for 'exhentai.py', but it improves
efficiency when the adapter is reused

gallery_dl/downloader/http.py

@@ -182,7 +182,11 @@ class HttpDownloader(DownloaderBase):
             # check for invalid responses
             validate = kwdict.get("_http_validate")
             if validate and self.validate:
-                result = validate(response)
+                try:
+                    result = validate(response)
+                except Exception:
+                    self.release_conn(response)
+                    raise
                 if isinstance(result, str):
                     url = result
                     tries -= 1