Check the error returned by genManifest

[bkreitch]

Check the error returned by genManifest function when three way merge is performed. This fixes the cases when chart produces incorrect manifest (for example a Deployment without image in initContainers):

...
initContainers:
  - name: nginx-init
...

Currently manifest with an error is just being dropped from comparison and appears as being removed in the diff:

$ helm diff upgrade dummy . --three-way-merge

default, dummy, Deployment (apps) has been removed:
- apiVersion: apps/v1
- kind: Deployment
- metadata:
-   creationTimestamp: "2022-10-21T08:22:50Z"
-   labels:
-     app.kubernetes.io/instance: dummy
-     app.kubernetes.io/managed-by: Helm
...

Now with the fix it will display the proper error:

Error: unable to generate manifests: cannot patch "dummy" with kind Deployment: Deployment.apps "dummy" is invalid: spec.template.spec.initContainers[0].image: Required value
Error: plugin "diff" exited with error

[bkreitch]

@databus23 Hi, could you please review it?

[databus23]

Thanks!