Tets suite: capture needed-package-updates's output by on failure

The output (i.e. what the problem is) will be included in the
exception's message so it actually ends up in the test suite logs.
The previously used fatal_system() doesn't capture the output at all,
and cucumber only catches stdout while stderr is passed through so
it's written to the terminal and thus doesn't end up in the test suite
logs at all.

Follow-up on tails/tails#14728, tails/tails!1155.