fix(client): added a guard to avoid the client to panic

[fredbi]

The client used to panic when hitting an unregistered producer.

This now returns an error as we expect clients to explicitly register their content producers.

• contributes generate client panic when getting a producer for multipart/form-data go-swagger/go-swagger#3192

Change type

Please select: 🆕 New feature or enhancement|🔧 Bug fix'|📃 Documentation update

Short description

Fixes

Full description

Checklist

• I have signed all my commits with my name and email (see DCO. This does not require a PGP-signed commit
• I have rebased and squashed my work, so only one commit remains
• I have added tests to cover my changes.
• I have properly enriched go doc comments in code.
• I have properly documented any breaking change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.36%. Comparing base (01f5c55) to head (9669745).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #474      +/-   ##
==========================================
+ Coverage   83.30%   83.36%   +0.05%     
==========================================
  Files          64       64              
  Lines        4516     4520       +4     
==========================================
+ Hits         3762     3768       +6     
+ Misses        583      582       -1     
+ Partials      171      170       -1     

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