Snapcraft should complain multiple `after` properties in a single part

[林博仁(Buo-ren, Lin)]

I stuck on debugging the missing patch files in the following part definition

```yaml
parts:
  picard:
    after:
    - patching

    source: https://github.com/metabrainz/picard.git
    source-depth: 100
    source-tag: release-2.1.3
    plugin: x-python
    override-build: |
      git apply --verbose "${SNAPCRAFT_STAGE}"/patching/localedir.patch

      snapcraftctl build
    after: [desktop-qt5]

  # Code patching to fix another part, when it cannot be solved by other means
  patching:
    source: patching
    plugin: dump
    organize:
      '*': patching/
    prime:
    - -*
```

Which I (veeeery) later recognize that it is the `after: [desktop-qt5]` line that cause the problem, and has the effect of Snapcraft no longer consider the `patching` part os the `picard` part's dependency.

I suggest that Snapcraft bails out prematurely if multiple `after` properties are defined in a single part.