brev create can fail silently when run without a tty

[myurasov-nv]

Problem

brev create launched non-interactively (no tty, output redirected) can fail silently: it prints nothing (not even an error) and no instance is ever created. The identical command re-run in the foreground works. Two occurrences on CLI v0.6.326 (macOS):

When (UTC)	Command	Result
2026-07-26 00:35	brev create <name> --type n1-standard-2:nvidia-tesla-t4:2 --startup-script @setup.sh --timeout 900, stdout to file, no tty	No output after 15+ min; instance never appeared in brev ls; foreground retry worked
2026-07-26 03:14	brev create <name> --type massedcompute_A100_sxm4_80G --startup-script @startup.sh --timeout 60, stdout to file, no tty	Same: silence, no instance; foreground retry worked

Both used --startup-script @file; unclear whether that is required to trigger it.

Impact

Automation cannot trust a create call: the only reliable signal is polling brev ls afterwards. A silent no-op wastes the whole readiness-wait timeout.

Fix

Whatever fails in the no-tty path should fail loudly (non-zero exit + message to stderr) instead of exiting silently without creating anything.