Log res.error() on all ServerTest ASSERT_TRUE(res) assertions

Only 8 of 423 call sites logged the actual error on failure. The
MbedTLS-backend flaky CI failure (connection-level ASSERT_TRUE(res),
~20-30ms) keeps landing on assertions without this diagnostic, so the
real error code has never been captured. Broadens the existing
GetWithRange-only logging (a4d7066) to every plain ASSERT_TRUE(res);
site, no behavior change.