Disambiguating HTTP-related failures in chemdbqueries tests

[timbernat]

Description

PubChemPy enforces a maximum "5 queries per second" limit in its PUGREST query API which this library frequently exceeds in CI matrix runs, causing false positive test failures due to dynamic throttling, rather than any issue with the code in this library. This PR aims to make response error handling and related tests aware of these shutoffs, and to handle these exceptions more gracefully to provide more helpful CI results

Todos

Notable points that this PR has either accomplished or will accomplish.

• chembdqueries.py sources
  • Make pubchem-agnostic HTTPError wrap retain context about why request failed
• test_chemdbqueries.py:
  • Catch and skip tests related to server disconnections (namely, codes 500, 503, and 504)

Questions

• Question1

Status

• Check CI passes
  • Attempt multiple runs before changes to see if errors are logged first (i.e. intentionally overload server requests and see if they can be detected in tests)