Actions formatting tightened up

Charm now expects a much simpler format for actions.yaml:

```text
<action name>:
  [description]: <string>
  [params]:
    <param 1>: <JSON-schema>
    <param 2>: <JSON-schema>
    ...
  [required]: <string list>
  [other keys e.g. "definitions"]: ...
<next action name>: ...
```
```go
if !c.Check(!failed, gc.Equals, t.shouldFail) {
  continue
}
```

I prefer seeing all failures at once, rather than stopping at each one.