doctor: throw a better error on fail in roachtests

**Describe the problem**
The output of `debug doctor examine cluster` can fail with a non-descriptive error:

```
run_154213.137443734_n1_cockroach-debug-doct: 15:42:13 cluster.go:2189: > ./cockroach debug doctor examine cluster --certs-dir certs
ERROR: current transaction is aborted, commands ignored until end of transaction block
SQLSTATE: 25P02
Failed running "debug doctor examine cluster"
run_154213.137443734_n1_cockroach-debug-doct: 15:42:13 cluster.go:2196: > result: COMMAND_PROBLEM: exit status 1
```

**To Reproduce**

This happens in the schemachange/mixed-versions roachtest on the new mixed-version framework when there is no restriction on `NumUpgrades` (i.e. if a test plan shows that we will upgrade from 22.2 to current - 23.2).

**Expected behavior**
We should have a more verbose error message on a failure like this.

Jira issue: CRDB-33653

Epic CRDB-60947