Fwdmachine fix and doc

[Lastpixl]

This PR:

• fixes a type error when writing a certificate to disk in ForwardMachine
• fixes the documentation of the ForwardMachine class to match the current implementation (which expects derived classes to raise exceptions rather than return values)

Thanks for considering it for merging.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.98%. Comparing base (b5ebda1) to head (af2f8b8).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/fwdmachine.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4823      +/-   ##
==========================================
- Coverage   80.98%   80.98%   -0.01%     
==========================================
  Files         365      365              
  Lines       89112    89112              
==========================================
- Hits        72167    72165       -2     
- Misses      16945    16947       +2     

Files with missing lines	Coverage Δ
scapy/fwdmachine.py	0.00% <0.00%> (ø)

... and 6 files with indirect coverage changes

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[gpotter2]

Hi and thanks for the PR ! You're right, there is more up to date documentation over https://scapy.readthedocs.io/en/latest/advanced_usage/fwdmachine.html but the docstring was out date.