RF: debug session only in the case of failure

Author: yarikoptic

appveyor.yml

@@ -31,7 +31,7 @@ test_script:
   - nosetests --with-doctest nibabel
 
 
-on_finish:
+on_failure:
   # enable the next to let the build VM block for up to 60min to log in via RDP and debug
   - ps: $blockRdp = $true; iex ((new-object net.webclient).DownloadString('https://raw.githubusercontent.com/appveyor/ci/master/scripts/enable-rdp.ps1'))