[FLAPI-2083] Fix HTTP client API error usage

[jesse-c]

The arguments are now the appropriate header list (though empty) and error (as a list).

The message argument is removed too as we implicitly get it from the first error.

[sgerrand]

LGTM, no blockers. 👍

[sgerrand]

💭 We should probably add unit tests for this class and others in this file at some future point.

[sgerrand]

Looks correct locally:

$ npx pyright duffel_api/http_client.py
No configuration file found.
pyproject.toml file found at .
Loading pyproject.toml file at ./pyproject.toml
Assuming Python platform Darwin
Auto-excluding **/node_modules
Auto-excluding **/__pycache__
Auto-excluding .git
stubPath ./typings is not a valid directory.
Search paths for .
  ./node_modules/pyright/dist/typeshed-fallback/stdlib
  .
  ./typings
  ./node_modules/pyright/dist/typeshed-fallback/stubs/...
  ~/.pyenv/versions/3.6.15/lib/python3.6
  ~/.pyenv/versions/3.6.15/lib/python3.6/lib-dynload
  ~/.pyenv/versions/3.6.15/envs/duffel-api-python-pyright/lib/python3.6/site-packages
Searching for source files
Found 1 source file
./duffel_api/http_client.py
  ./duffel_api/http_client.py:4:6 - warning: Import "requests" could not be resolved from source (reportMissingModuleSource)
  ./duffel_api/http_client.py:5:6 - warning: Import "requests" could not be resolved from source (reportMissingModuleSource)
0 errors, 2 warnings, 0 infos 
Completed in 0.692sec