Add debug option for on-error

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#### Description

Allow debug access to build only when a step has an error.

#### Use Case(s)

When running packer with `-debug` a PEM file is created to allow SSH into the instance for debugging, however you have to monitor the build and step through each step to get where you need to be before the build is paused and you can use the PEM file to SSH into the instance. This is great for general debugging, however if you have a single problem that occurs much later in build you have to manually step through each step.

Adding a `debug` option to `on-error` can do exactly the same, but only after any step fails.

So when the step fails, a PEM is written to disk and steps paused to allow debugging.

#### Potential configuration

```
packer build -on-error=abort packer.file
```

#### Potential References

* [Debugging Packer Builds](https://developer.hashicorp.com/packer/docs/debugging)