render: Don't show the details of the errors raised by GStreamer pipelines

They're useless for users as they're a string of the path/hierarchy of objects,
and they'll never be enough information to troubleshoot the issue by themselves.

Fixes bug #739832
docs/release: Correct the tarball download URLs
pitivi-git-environment: Fix the comment on how to run "master"

Also remove redundant code that was handled elsewhere