Packer keeps trying to pull a non existent source image

#### Overview of the Issue

I've noticed that one Packer CI job was running for a very long time: more than 30 minutes, when the average job's duration is less than 10 minutes. The following message was logged over and over again:

```
openstack.debian: Image not Active, retrying in 10 seconds
```

I killed the job and found the issue: the image declared in the `external_source_image_url` variable did not exist: the HTTP endpoint was returning a 404 error.

#### Reproduction Steps

Set the `external_source_image_url` variable to a non existent image.

### Plugin and Packer version

Packer 1.7.8
Openstack plugin 1.0.0

### Simplified Packer Buildfile

```
packer {
required_version = "1.7.8"
required_plugins {
openstack = {
version = "1.0.0"
source = "github.com/hashicorp/openstack"
}
}
}

source "openstack" "debian" {
cloud = "dc1"
external_source_image_format = "raw"
external_source_image_url = "http://cdimage.debian.org/cdimage/openstack/10.11.1-20211029/debian-10.11.1-20211029-openstack-amd64.raw"
image_min_disk = "10"
image_name = "bla"
flavor = "packer"
ssh_username = "debian"
}

build {
sources = ["source.openstack.debian"]
}
```
### Operating system and Environment details

Debian Buster

### Log Fragments and crash.log files

```
$ packer build debian.pkr.hcl
116openstack.debian: output will be in this color.
117==> openstack.debian: Loading flavor: packer
118 openstack.debian: Verified flavor. ID: 121d18c3-f0e0-4096-ba60-54c4050ea109
119==> openstack.debian: Creating temporary keypair: packer_61d470f3-fd3e-a797-819b-21b31030b544 ...
120==> openstack.debian: Created temporary keypair: packer_61d470f3-fd3e-a797-819b-21b31030b544
121==> openstack.debian: Creating image using external source image with name packer_61d470f3-fcb1-75b4-1c99-251728853883
122==> openstack.debian: Using disk format raw
123==> openstack.debian: Created image with ID 4964814c-7512-4293-879d-50b5b25b37cb
124==> openstack.debian: Importing External Source Image from URL http://cdimage.debian.org/cdimage/openstack/10.11.1-20211029/debian-10.11.1-20211029-openstack-amd64.raw
125 openstack.debian: Image not Active, retrying in 10 seconds
126 openstack.debian: Image not Active, retrying in 10 seconds
127 openstack.debian: Image not Active, retrying in 10 seconds
128 openstack.debian: Image not Active, retrying in 10 seconds
129 openstack.debian: Image not Active, retrying in 10 seconds
130 openstack.debian: Image not Active, retrying in 10 seconds
131 openstack.debian: Image not Active, retrying in 10 seconds
132 openstack.debian: Image not Active, retrying in 10 seconds
```

No se nombran archivos ni pruebas. Empieza localizando el manejo de external_source_image_url y el bucle de reintento del estado de la imagen; después, reproduce el problema con una URL que devuelva 404. La solución debe incluir un fallo acotado o que termine, con un mensaje de error claro, en lugar de reintentos repetidos.

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