fix: report a clear error when chart-testing blob verification fails

[somaz94]

Under set -o errexit, the retVal=$? check after cosign verify-blob was
unreachable: a non-zero cosign verify-blob aborts the script immediately, so
the intended "Unable to validate chart-testing version" message never fired. The
error branch also called log_error, which is not defined anywhere in ct.sh
(it would fail with command not found if it were ever reached).

This rewrites the check as if ! cosign verify-blob ...; then, which suspends
errexit for the tested command so the failure branch is actually reached, and
replaces the undefined log_error with echo "ERROR: ..." >&2, matching the
existing error style used in parse_command_line. Behavior on success is
unchanged; on verification failure the script now prints the intended clear
error and exits 1.

Validation:

• bash -n ct.sh and zsh -n ct.sh parse clean
• shellcheck ct.sh clean
• Confirmed in both bash and zsh that if ! cmd reaches the failure branch under set -o errexit

Note: open PR #205 wraps this same block in a verify_blob toggle but copies the
broken retVal/log_error pattern verbatim, so this fix is complementary
regardless of merge order.