RE: Bug in ALTER SUBSCRIPTION ... SERVER / ... CONNECTION with broken old server

Dear Jeff,

I went through the 0003 patch. Apart from HINT message, I found that error code
ERRCODE_CONNECTION_FAILURE is always used. Should we report the original errcode
of the root cause? Or it's OK because the observed outcome is the cannot-connect
error?

Another comment:

```
+ conninfo = ForeignServerConnectionString(subowner, server);
+ Assert(conninfo != NULL);
```

Do we have to consider the case that ForeignServerConnectionString() returns NULL
without any ERRORs? For the production build, the Assert() would be skipped and
the segmentation fault would happen in the `if (!conninfo)` part.
I checked the doc [1] but there are no specifications for it.

[1]: https://www.postgresql.org/docs/devel/sql-createforeigndatawrapper.html#:~:text=ereport(ERROR)%20function.-,CONNECTION%20connection_function,-connection_function%20is%20the

Best regards,
Hayato Kuroda
FUJITSU LIMITED