Fall back to public PyPI if 'ConnectionError' occurs

• I have searched the issues of this repo and believe that this is not a duplicate.
• I have searched the documentation and believe that my question is not covered.

Feature Request

I have a private PyPI repository defined in pyproject.toml. The repository is located within my corporate network. When I don't use VPN and try to install any package, I get such an error:

  ConnectionError

  HTTPConnectionPool(host=SOME_HOST, port=80): Max retries exceeded with url: SOME_URL (Caused by NewConnectionError('<urllib3.connection.HTTPConnection object at 0x109f17f10>: Failed to establish a new connection: [Errno 8] nodename nor servname provided, or not known'))

which is quite obvious. But what I would like to see is an option to ignore the error and try to install packages from the next expected repository, in that case - the public PyPI. Currently, the fallback mechanism works only for cases when a repository is reachable, but some package is missing.

PS The same error occurs even if secondary = true is used - which of course is reasonable, but it's not always the behaviour you would like to achieve.

Start by tracing Poetry's existing repository fallback behavior and how ConnectionError is handled during package installation. Compare the current behavior for a reachable repository with a missing package to the requested behavior for an unreachable private repository, then verify that installation can continue against public PyPI without breaking the existing secondary-repository behavior.