feat: report xml2rfc errors for submissions

[jennifer-richards]

This should help with #5696 and other issues related to opaque / unhelpful feedback when a submission fails.

The main improvement here is to fix a bug when calling prep.prep(). Previously, the code checked whether its result was None and attempted to pass prep.errors back in a way that shows up on the submission history. This didn't work because prep.prep() raises an exception if prep.errors is not empty rather than returning None. With that fixed, the list of errors should appear on the submission page as intended.

Before diagnosing that detail, I'd started down the path of capturing the output from xml2rfc with the intention of making that available to the submitter. That might be useful but it is also likely to reveal internal filesystem paths. Thus, instead of making it available, for now I'm just logging it. We can review this to decide whether it's useful and what sort of sanitizing will be needed.

[codecov]

Codecov Report

Attention: Patch coverage is 57.44681% with 20 lines in your changes missing coverage. Please review.

Project coverage is 88.88%. Comparing base (c7f6bde) to head (98d733b).
Report is 104 commits behind head on main.

Files with missing lines	Patch %	Lines
ietf/submit/utils.py	57.44%	20 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8000      +/-   ##
==========================================
+ Coverage   88.78%   88.88%   +0.09%     
==========================================
  Files         296      304       +8     
  Lines       41320    41313       -7     
==========================================
+ Hits        36687    36720      +33     
+ Misses       4633     4593      -40     

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