fix: remove infinite while loop and added error handling for retrieve_cert()

[mowdash]

Working with TPP, trying to retrieve a non-existing cert would result in an infinite while loop with the following error message...

ERROR:root:Unknown error format: {'Error': 'Certificate test does not exist.'}
ERROR:root:Unknown error format: {'Error': 'Certificate test does not exist.'}
ERROR:root:Unknown error format: {'Error': 'Certificate test does not exist.'}

Instead we now log the error once and raise a RetrieveCertificateNotFound error.