Currently the Authenticate.storeIdentityProfile() method always emits an
Error log. If there is no error from cookieChunker.SetCookie(), this
results in an empty log entry:
{"level":"error","time":"2023-06-27T23:56:38Z"}
Refactor this method to instead return the error from SetCookie(), and
update the calling code so that it logs a message only when this error
is non-nil.
(Moving the log call to the calling method gives access to the request
context, so the log entry will include the request ID and other related
info.)
