fix: handle non-existing cargo packages

[loispostula]

Fix #1459

Make sure we can handle the cases when a package does not exists

Test

To test this pull request, you can run check the following pipeline file

name: Retrieve the latest Cargo Package Version

scms:
  private-registry:
    kind: git
    spec:
      url: "https://github.com/updatecli-test/fake-cargo-registry.git"
      branch: "main"

conditions:
  existing-package-private-reg:
    name: Test if test-crate-2 0.1.0 exists on private registry
    kind: cargopackage
    scmid: private-registry
    spec:
      package: test-crate-2
      version: 0.1.0
  existing-package-not-existing-version-private-reg:
    name: Test if test-crate-2 0.6.0 does not exists on private registry
    kind: cargopackage
    scmid: private-registry
    failwhen: true
    spec:
      package: test-crate-2
      version: 0.6.0
  non-existing-package-private-reg:
    name: Test if non-existing-test-crate 0.1.0 does not exists on private registry
    kind: cargopackage
    scmid: private-registry
    failwhen: true
    spec:
      package: non-existing-test-crate
      version: 0.1.0
  existing-package-public-reg:
    name: Test if rand version exists on public reg
    kind: cargopackage
    spec:
      package: rand
      version: 0.7.2
  existing-package-p-not-existing-version-public-reg:
    name: Test if rand version 99.99.99 does not exists on public reg
    kind: cargopackage
    failwhen: true
    spec:
      package: rand
      version: 99.99.99
  non-existing-package-public-reg:
    name: Test if non-existing-package-to-be-sure-123456 does exists on public reg
    kind: cargopackage
    failwhen: true
    spec:
      package: non-existing-package-to-be-sure-123456
      version: 0.7.2

[olblak]

I faced the same condition problem for other plugin.
I don't reminder if it is unified but I think it is the right approach