Safer exception handling in 'process_recipe' service

[tieneupin]

We discovered a bug in the exception handling logic of the process_recipe service when it tries and fails to look for an existing recipe. It will try to reject the message with transport=rw.transport, which will error if the recipe wrapper variable is set to None.

This PR makes the exception handling logic safer by letting the service default to using the Transport class set by self._transport when rejecting the message.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 83.21%. Comparing base (cdcc32e) to head (c9399df).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #281   +/-   ##
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  Coverage   83.21%   83.21%           
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  Files          75       75           
  Lines        9437     9437           
  Branches     1276     1276           
=======================================
  Hits         7853     7853           
  Misses       1088     1088           
  Partials      496      496           

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