Use flint_vsnprintf in FLINT error handling

[lgoettgens]

instead of letting FLINT print to stdout and capturing that.

Unfortunately, we have to guess a maximum length to allocate the formatted message string. The classic approach of calling vsnprintf twice, once with a NULL buffer to obtain the length as the return value and then a second time with an allocated buffer to the exact size, does not work here, as it requires us to copy the va_list. And I don't really see a way to do that from julia.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.25%. Comparing base (3452249) to head (c02dfbc).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2317      +/-   ##
==========================================
- Coverage   88.28%   88.25%   -0.03%     
==========================================
  Files         102      102              
  Lines       37842    37833       -9     
==========================================
- Hits        33408    33389      -19     
- Misses       4434     4444      +10     

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