prevent unhandled exception on Cloudflare challenges (#868)

The relatively new v2 challenges aren't supported (*), but retrying
often enough may yield a v1 challenge which can be solved.

(*) and probably never will. They are far too complicated to do without
a real browser.

gallery_dl/cloudflare.py

@@ -32,23 +32,11 @@ def solve_challenge(session, response, kwargs):
     """Solve Cloudflare challenge and get cfclearance cookie"""
     parsed = urllib.parse.urlsplit(response.url)
     root = parsed.scheme + "://" + parsed.netloc
-
     page = response.text
-    try:
-        params = {"ray": text.extract(page, '?ray=', '"')[0]}
-
-        url = root + "/cdn-cgi/images/trace/jschal/nojs/transparent.gif"
-        session.request("GET", url, params=params)
-
-        url = root + "/cdn-cgi/images/trace/jschal/js/nocookie/transparent.gif"
-        session.request("GET", url, params=params)
-    except Exception:
-        pass
 
     cf_kwargs = {}
     headers = cf_kwargs["headers"] = collections.OrderedDict()
     params = cf_kwargs["data"] = collections.OrderedDict()
-    url = root + text.unescape(text.extract(page, 'action="', '"')[0])
     headers["Referer"] = response.url
 
     form = text.extract(page, 'id="challenge-form"', '</form>')[0]
@@ -58,12 +46,27 @@ def solve_challenge(session, response, kwargs):
         if not name:
             continue
         if name == "jschl_answer":
-            value = solve_js_challenge(page, parsed.netloc)
+            try:
+                value = solve_js_challenge(page, parsed.netloc)
+            except Exception:
+                return response, None, None
         else:
             value = element.attrib.get("value")
         params[name] = value
 
+    try:
+        params = {"ray": text.extract(page, '?ray=', '"')[0]}
+
+        url = root + "/cdn-cgi/images/trace/jschal/nojs/transparent.gif"
+        session.request("GET", url, params=params)
+
+        url = root + "/cdn-cgi/images/trace/jschal/js/nocookie/transparent.gif"
+        session.request("GET", url, params=params)
+    except Exception:
+        pass
+
     time.sleep(4)
+    url = root + text.unescape(text.extract(page, 'action="', '"')[0])
     cf_response = session.request("POST", url, **cf_kwargs)
 
     if cf_response.history: