Redirect httpbin error output to the log file too

This can be at least as useful as stdout.

appveyor.yml

@@ -75,7 +75,7 @@ before_test:
 - ps: |
     Write-Output "Getting and launching httpbin."
     C:\Python35\Scripts\pip.exe --disable-pip-version-check --no-warn-script-location install httpbin
-    Start-Job -Name wx_httpbin { C:\Python35\python.exe -m httpbin.core > c:\projects\wxwidgets\httpbin.log }
+    Start-Job -Name wx_httpbin { C:\Python35\python.exe -m httpbin.core 2>&1 > c:\projects\wxwidgets\httpbin.log }
     curl http://127.0.0.1:5000/ip
     $env:WX_TEST_WEBREQUEST_URL="http://127.0.0.1:5000"