bug#25236: 26.0.50; sql-postgres not working on master

[Simen Heggestøyl]

This bug seems to be caused by a change I made in
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=24646, making
`sql-postgres` able to complete the database name by looking at the
result of `psql -ltX`.

psql exits with status code 2 if the connection to the server went
bad, for instance if permissions are missing [1].

I think we can fix this by being more lax in
`sql-postgres-list-databases`, and just don't provide any completions
should the command fail. Then we'd also need to change the value of
the `REQUIRE-MATCH` parameter to `completing-read` on line 2966, since
requiring a match when there are no completions would make it
impossible to continue.

What do you think, Michael?

-- Simen

[1] 
https://www.postgresql.org/docs/current/static/app-psql.html#AEN98634