PyLabs: Don't ignore 'inject-as-head' exit code

Related to mail-conversations with Wouter.

CI passed for -shaky (at least the 2 changed tests, others still running), which is the only impacted testsuite:

```
test_inject_as_head : asserts shortcut for those who don't want to collapse periodically. (eta: 20 s) ... ok
Test inject_as_head with a fake database file (should fail) ... Fatal error: exception Failure("read error")
ok
```

Notice this goes into 1.6 (will forward-merge to 1.7 afterwards).