Edge case not handled properly in transformer sidecar

[ponyisi]

Describe the bug
An UnboundLocalError can result in transformer.py of the transformer sidecar

To Reproduce
Difficult to reproduce in the wild, but one can see that in callback(), if the test on line 269 fails for whatever reason, it breaks out of the particular replica attempt without setting transformer_stats. The variable may therefore not be set when reaching the error reporting:

ServiceX/transformer_sidecar/src/transformer_sidecar/transformer.py

Line 308 in e0d5fa1

"error-info": transformer_stats.error_info,

Expected behavior
A more meaningful error is put in the logfile than UnboundLocalError

Additional context
Reported by Alkaid Cheng for a transformation with the python codegen. Succeeded on second attempt (presumably a transient issue).