Support function resolution in multiple namespaces

[LucasRoesler]

Description

What

• Ensure that the pass only the function name to the Endpoint lister,
  because the namespace is already set when constructing the lister.
  The existing code would always return a 404 because it sent the
  "function_name.namespace" as the function name. This can never be
  found since it is not a valid service name
• Update the error messages to make the component parts of the message
  easier to read. It will now be easier to see which part is the error from the
  k8s api.

Motivation and Context

• I have raised an issue to propose this change (required)

Resolves openfaas/faas#1565

How Has This Been Tested?

The unit test for the proxy resolver has been updated to catch this case and it now passes.

It has also been tested locally using

kind create cluster

# doing this will cause extras and openfaas-fn to show in the UI
kubectl create ns extras
kubectl annotate ns openfaas-fn  openfaas=1
kubectl annotate ns extras  openfaas=1

arkade install openfaas --basic-auth=false --direct-functions=false --operator --clusterrole --set operator.image=theaxer/faas-netes:6802054

# in a separate terminal
kubectl port-forward svc/gateway 8080

open http://localhost:8080 and use the UI to deploy and invoke the function

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have signed-off my commits with git commit -s
• I have added tests to cover my changes.
• All new and existing tests passed.

[amalkh5]

@LucasRoesler I've tested these changes and it working as expected.

[LucasRoesler]

Thanks 💪

[alexellis]

Thank you @amalkh5

[alexellis]

Approved