[socks] Report errors elegantly when credentails are required but missing

In some non-standard implementations, the server may respond AUTH_USER_PASS
even if's not listed in available authentication methods. (it should
respond AUTH_NO_ACCEPTABLE per standards)

ChangeLog

@@ -1,5 +1,9 @@
 version <unreleased>
 
+Core
+
+* [socks] Handle cases where credentials are required but missing
+
 Extractors
 - [azubu] Remove extractor (#12813)
 * [porn91] Fix extraction (#12814)

youtube_dl/socks.py

@@ -193,9 +193,10 @@ class sockssocket(socket.socket):
 
         self._check_response_version(SOCKS5_VERSION, version)
 
-        if method == Socks5Auth.AUTH_NO_ACCEPTABLE:
+        if method == Socks5Auth.AUTH_NO_ACCEPTABLE or (
+                method == Socks5Auth.AUTH_USER_PASS and (not self._proxy.username or not self._proxy.password)):
             self.close()
-            raise Socks5Error(method)
+            raise Socks5Error(Socks5Auth.AUTH_NO_ACCEPTABLE)
 
         if method == Socks5Auth.AUTH_USER_PASS:
             username = self._proxy.username.encode('utf-8')