Don't die if stderr/stdout do not support set_parent() #2925

If the user redirects the streams to a file for example,
the kernel would raise an exception because file
objects do not have set_parent defined, unlike ipython's
OutStream.
This happens even if the user does this in a spawned thread.
Merge pull request #2926 from y-p/GH2925

Don't die if stderr/stdout do not support set_parent()

If the user redirects the streams to a file for example,
the kernel would raise an exception because file
objects do not have set_parent defined, unlike ipython's
OutStream.
This happens even if the user does this in a spawned thread.

closes #2925