⚠️ check return err for customized agent install namespace

[zhujian7]

Summary

This PR changes the RegistrationOption interface:
from

AgentInstallNamespace func(addon *addonapiv1alpha1.ManagedClusterAddOn) string

to

AgentInstallNamespace func(addon *addonapiv1alpha1.ManagedClusterAddOn) (string, error)

we need this error info to determine whether the namespace is empty or an error occurred.

Related issue(s)

Fixes #

[zhujian7]

/hold

[zhujian7]

/assign @qiujian16
/unhold

[qiujian16]

/approve
/lgtm

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: qiujian16, zhujian7

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[qiujian16]

/cherrypick release-0.9

[openshift-cherrypick-robot]

@qiujian16: #257 failed to apply on top of branch "release-0.9":

Applying: check return err for customized agent install namespace
Using index info to reconstruct a base tree...
M	pkg/addonfactory/addonfactory.go
M	pkg/addonfactory/helm_agentaddon.go
M	pkg/addonfactory/template_agentaddon.go
M	pkg/agent/inteface.go
Falling back to patching base and 3-way merge...
Auto-merging pkg/agent/inteface.go
Auto-merging pkg/addonfactory/template_agentaddon.go
Auto-merging pkg/addonfactory/helm_agentaddon.go
CONFLICT (content): Merge conflict in pkg/addonfactory/helm_agentaddon.go
Auto-merging pkg/addonfactory/addonfactory.go
CONFLICT (content): Merge conflict in pkg/addonfactory/addonfactory.go
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 check return err for customized agent install namespace
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

In response to this:

/cherrypick release-0.9

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