Add lookup of underenhet by org number

[jodal]

• Drop fixtures for loading mock responses
• Make room for get_underenhet()
• Extract error handling from client methods
• Add get_underenheter() method

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (fde80f7) 92.59% compared to head (9cf1dd6) 92.95%.

Files	Patch %	Lines
src/brreg/enhetsregisteret/_client.py	75.00%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #11      +/-   ##
==========================================
+ Coverage   92.59%   92.95%   +0.36%     
==========================================
  Files           5        5              
  Lines         108      142      +34     
  Branches        4        8       +4     
==========================================
+ Hits          100      132      +32     
- Misses          6        7       +1     
- Partials        2        3       +1     

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