misc fixes

[abhidg]

• fix: raise Error, otherwise returns None
• Add some type hints, fix typo
• resources: use set.intersection and remove if statement

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.46154% with 3 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (main@2d2c5ad). Click here to learn what that means.

Files	Patch %	Lines
fhirflat/resources/base.py	80.00%	1 Missing ⚠️
fhirflat/resources/location.py	50.00%	1 Missing ⚠️
fhirflat/resources/organization.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #27   +/-   ##
=======================================
  Coverage        ?   94.54%           
=======================================
  Files           ?       35           
  Lines           ?     1228           
  Branches        ?        0           
=======================================
  Hits            ?     1161           
  Misses          ?       67           
  Partials        ?        0           

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[pipliggins]

Thanks!