adding detailed-exitcode

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#### Description

It would be nice addition to add additional parameter with detailed exit code, something like: `-detailed-exitcode`

This will be returning standard 0-1 codes but also additional which will be showing case when packer run partially or when it was not able to make build because for example AWS AMI with such name already existing.

#### Use Case(s)

I am using packer plugin github.com/ethanmdavidson/git which helps me manage version of builds. I want to cover most of CI/CD process natively in packer. So now I am using following versioning code:

```hcl
locals {
truncated_sha = substr(data.git-commit.cwd-head.hash, 0, 8)
version = data.git-repository.cwd.head == "master" && data.git-repository.cwd.is_clean ? var.version : "${var.version}-${local.truncated_sha}"
}

data "git-repository" "cwd" {}

data "git-commit" "cwd-head" {}
```

Currently when AMI was already build using amazon-ebs I get error:

```
........
==> jenkins.amazon-ebs.jenkins: Error: AMI Name: 'jenkins-1.0.0-62269622' is used by an existing AMI: ami-07025404d34e95d3e
Build 'jenkins.amazon-ebs.jenkins' errored after 3 seconds 514 milliseconds: Error: AMI Name: 'jenkins-1.0.0-62269622' is used by an existing AMI: ami-07025404d34e95d3e

==> Wait completed after 3 seconds 514 milliseconds

==> Some builds didn't complete successfully and had errors:
--> jenkins.amazon-ebs.jenkins: Error: AMI Name: 'jenkins-1.0.0-62269622' is used by an existing AMI: ami-07025404d34e95d3e
```

it would be nice to be able recognise using exit code that build was not done because arleady existing or in case of multiple AMIs build that was completed partially

#### Potential configuration

0 = Succeeded with all build jobs
1 = Error with all builds
2 = Succeeded partially
3 = Builds were not proceeded because they already existing

#### Potential References
https://www.terraform.io/cli/commands/plan#detailed-exitcode