fix: return HTTP 400 if key unmarshal fails

[nipsufn]

This should correct behavior where on key unmarshal error application returns HTTP 500.

Related issue(s)

Checklist

• I have read the contributing guidelines.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I am following the
  contributing code guidelines.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got the approval (please contact
  [email protected]) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added or changed the documentation.

Further Comments

[codecov]

Codecov Report

Merging #3594 (6b2ce92) into master (6a0a914) will increase coverage by 0.01%.
The diff coverage is n/a.

❗ Current head 6b2ce92 differs from pull request most recent head 170f312. Consider uploading reports for the commit 170f312 to get more accurate results

@@            Coverage Diff             @@
##           master    #3594      +/-   ##
==========================================
+ Coverage   78.22%   78.24%   +0.01%     
==========================================
  Files         341      341              
  Lines       23141    23098      -43     
==========================================
- Hits        18103    18073      -30     
+ Misses       3681     3673       -8     
+ Partials     1357     1352       -5     

see 3 files with indirect coverage changes

[hperl]

Nice fix! Just a small comment.