Sensitive connection string information part of parse error

The error `pgconn.parseConfigError` returned from `pgconn.ParseConfig` includes the full connection string in plain text (including the provided password).
Sensitive information, like the password, should not be visible in plaintext.

## Example:
```go
connString := "postgresql://postgres:Hello121\\@127.0.0.1:5432/postgres"
_, err := pgconn.ParseConfig(connString)
fmt.Println(err.Error())
```

Result:
```text
cannot parse `postgresql://postgres:xxxxxx@127.0.0.1:5432/postgres`: failed to parse as URL (parse "postgresql://postgres:Hello121\\@127.0.0.1:5432/postgres": net/url: invalid userinfo)
```

The above described incident can easily happen, if one passes the connection string via an environment variable with a password that is not url encoded.

As matter of effect, if the returned error gets logged in a production environment, the connection string, including the provided password, gets stored in the logging backend.

e.g.:
```shell
POSTGRES_PASSWORD='not_\_url_/encoded_|password'
/my-service --conn-string "postgresql://postgres:${POSTGRES_PASSWORD}@127.0.0.1:5432/postgres"
```

Start at pgconn.ParseConfig and follow how pgconn.parseConfigError formats failures from invalid connection strings. Reproduce the example and verify that parse errors no longer expose the supplied password or full connection string, including the environment-variable case.

Written by the indexing model from the issue text.