F fix timeout error messages

Summary

For #215 (closed) (https://gitlab.com/linkahead/linkahead-pylib/-/issues/87). Fixes an issue whereby the HTTPServerError class would have too strong assumptions regarding the validity and the contents of the XML response. This would cause problems especially when encountering proxy or other network timeouts.

Focus

This doesn't extend any error handling, it simply ensures that the error can be created correctly.

Test Environment

New unit tests should be sufficient. You can also provoke the original bug by starting LinkAhead with envoy_enabled: true and envoy_rout_timeout set to something very low, e.g., 5 s. Then, I could provoke the original bug by inserting a container with 5000 entities.

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