Simplify message around nsjail error

We have known problems (docker; distros w/o user namespaces) which mean
that we won't be turning this into a fatal error anytime soon. Also
remove the bug report link, it's not useful to continue getting the same
reports.

Bug: 123210688
Test: treehugger
Change-Id: I271871d68150417ac938f074d3730cad4518e327
Merged-In: I271871d68150417ac938f074d3730cad4518e327
(cherry picked from commit 1871d88e)