Report teardown-only failures through the exit status and the log

[bgrewell]

Stacked on #99.

The problem

A cleanup job that cannot report its own failure is worse than no cleanup job. Two independent defects made --teardown-only exactly that:

1. It returned nil unconditionally. A broken teardown step, or a node that would not come down, exited 0 and CI moved on with resources still allocated.
2. Its errors bypassed the formatter. All three messages were written with fmt.Printf straight to stdout rather than through the formatter --log wraps, so a -l file contained the task lines and none of the errors.

The documented workaround was to pipe the console through tee and grep it — and that recipe was itself wrong, since a trailing grep … && exit 1 as a script's last line fails the job precisely when cleanup succeeded.

The fix

Failures are collected as the run proceeds, printed through the formatter, and summarized in an error that sets the exit status:

Error: teardown failed: 1 of 4 cleanup operations did not complete;
the first was: running teardown step "drop test network": command failed with exit code 7

Best-effort behaviour is deliberately unchanged — every teardown step, node, and platform is still attempted after an earlier failure, because leaving resources behind is worse than a partial teardown. Only the reporting changed.

Verified end to end:

Case	Exit	-l captures the error
Failing teardown step + failing node teardown	1	yes
Clean teardown	0	n/a

and the later steps still ran in the failing case.

The CI recipe collapses

dart -c suite.yaml --teardown-only     # non-zero if any cleanup failed

No tee, no grep, no exit-status normalisation. The docs section that explained the scraping workaround is replaced by that one line.

Verification

New controller tests assert the failing case exits non-zero, that cleanup still continues past the failure (both the later step and node teardown run), and that the errors reach the formatter rather than raw stdout — plus that a clean run still exits 0 and prints no errors. The test formatter now records PrintError calls, which is what makes the second assertion possible.

go build, go vet, gofmt, and the full suite are green; the docs site builds.