Handle DNS failure for repository probes

[leafty]

Handle any http connection error as a missing resource error for probing repositories.

Logs:

Srv 0 2025-01-21 09:41:15 +0000 ERROR: Exception occurred while handling uri: 'https://renku-ci-ui-3491.dev.renku.ch/api/data/repositories/http%3A%2F%2Ffoobar/probe'
Traceback (most recent call last):
  File "/app/env/lib/python3.12/site-packages/httpx/_transports/default.py", line 101, in map_httpcore_exceptions
    yield
  File "/app/env/lib/python3.12/site-packages/httpx/_transports/default.py", line 394, in handle_async_request
    resp = await self._pool.handle_async_request(req)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/env/lib/python3.12/site-packages/httpcore/_async/connection_pool.py", line 256, in handle_async_request
    raise exc from None
  File "/app/env/lib/python3.12/site-packages/httpcore/_async/connection_pool.py", line 236, in handle_async_request
    response = await connection.handle_async_request(
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/env/lib/python3.12/site-packages/httpcore/_async/connection.py", line 101, in handle_async_request
    raise exc
  File "/app/env/lib/python3.12/site-packages/httpcore/_async/connection.py", line 78, in handle_async_request
    stream = await self._connect(request)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/env/lib/python3.12/site-packages/httpcore/_async/connection.py", line 124, in _connect
    stream = await self._network_backend.connect_tcp(**kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/env/lib/python3.12/site-packages/httpcore/_backends/auto.py", line 31, in connect_tcp
    return await self._backend.connect_tcp(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/env/lib/python3.12/site-packages/httpcore/_backends/anyio.py", line 113, in connect_tcp
    with map_exceptions(exc_map):
         ^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.12/contextlib.py", line 158, in __exit__
    self.gen.throw(value)
  File "/app/env/lib/python3.12/site-packages/httpcore/_exceptions.py", line 14, in map_exceptions
    raise to_exc(exc) from exc
httpcore.ConnectError: [Errno -2] Name or service not known