Fix HTTP error code on a free-busy-report with invalid range

[alecpl]

Fixes #992.

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 97.24%. Comparing base (ed901a9) to head (c17b2cc). :warning: Report is 14 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1609      +/-   ##
============================================
+ Coverage     97.23%   97.24%   +0.01%     
- Complexity     2836     2837       +1     
============================================
  Files           175      175              
  Lines          9028     8854     -174     
============================================
- Hits           8778     8610     -168     
+ Misses          250      244       -6     

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