[JUJU-3456] Fix returned error when yaml is invalid on diff-bundle

[nvinuesa]

When dealing with invalid yaml in local bundle, the error was treated as if the file was not present, therefore continuing to treat the input as a charm bundle url.

This patch fixes the returned error if it's a NotValid error.

Checklist

• Code style: imports ordered, good names, simple structure, etc
• Comments saying why design decisions were made
• Go unit tests, with comments saying what you're testing
• Integration tests, with comments saying what you're testing
• doc.go added or updated in changed packages

QA steps

Having an empty juju model and this test.yaml bundle:

series: focal
applications:
  ubuntu:
    charm: ubuntu
    channel: stable
    revision: 21
    num_units: 1
    to:
    - "0"
    constraints: arch=amd64
va
machines:
  "0":
    constraints: arch=amd64

then you should get this error:

$ juju diff-bundle test.yaml
ERROR cannot unmarshal bundle contents: unmarshal document 0: yaml: line 12: could not find expected ':'

if you remove line 12 then:

$ juju diff-bundle test.yaml
applications:
  ubuntu:
    missing: model
machines:
  "0":
    missing: model

Bug reference

https://bugs.launchpad.net/juju/+bug/2015315

[SimonRichardson]

I'm unsure why we thought an invalid bundle.yaml should be interpreted and treated as a charm store charm. This is over 4-year-old code, I suspect the original assumption was incorrect or the underlying library has changed.

[nvinuesa]

/merge