fetchurl: disallow specifying both sha256 and hash

A full check would be more complicated to write -
and more importantly - probably also more expensive.

Motivation: eval-time catch for errors like in commit 8198636.

pkgs/build-support/fetchurl/default.nix

@@ -117,6 +117,9 @@ let
     else throw "fetchurl requires either `url` or `urls` to be set";
 
   hash_ =
+    # Many other combinations don't make sense, but this is the most common one:
+    if hash != "" && sha256 != "" then throw "multiple hashes passed to fetchurl" else
+
     if hash != "" then { outputHashAlgo = null; outputHash = hash; }
     else if md5 != "" then throw "fetchurl does not support md5 anymore, please use sha256 or sha512"
     else if (outputHash != "" && outputHashAlgo != "") then { inherit outputHashAlgo outputHash; }