Add missing import for SerializerError

[pko-trackunit]

Description

What behavior does this PR change, and why?

Fixes:

    except SerializerError as e:
           ^^^^^^^^^^^^^^^
NameError: name 'SerializerError' is not defined

Author Validation

Describe the validation already done, or needs to be done, by the PR submitter. Please note submitters are expected to build docs-platform locally to validate before merging from examples.

• clients/cloud

Reviewer Tasks

Describe the tasks/validation that the PR submitter is requesting to be done by the reviewer.

• clients/cloud

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}