Add custom ErrUnknownBodyFormat

[alecsammon]

Use a declared error for unknown body type to allow users of the library to detect the type of error.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5d958eb) 90.77% compared to head (1b09f0b) 90.77%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #91   +/-   ##
=======================================
  Coverage   90.77%   90.77%           
=======================================
  Files          33       33           
  Lines        1464     1464           
=======================================
  Hits         1329     1329           
  Misses         90       90           
  Partials       45       45           

Flag	Coverage Δ
unittests	90.77% <100.00%> (ø)

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