Reject unknown yaml tags

We often get issues where folks have typing an exclamation point in some yaml by accident, and the agent parses and silently ignores the problem. Most recently, https://github.com/buildkite/agent/issues/1281.

Syntax like this, which is a "master" yaml tag, should probably throw a syntax error or something:

```yaml
step
- branches: !master
commands: echo hello world
```

Start by tracing the agent's YAML parsing path for the example in this issue and compare its behavior with the related report in issue 1281. Reproduce the input containing `!master`, then verify that unknown YAML tags produce a syntax error instead of being silently ignored.

Written by the indexing model from the issue text.