Error message against iso-8859-1 key in pg_service.conf is unclear

Hi Hacker,

I noticed that, if I put a wrong kay name in pg_service.conf, the error message is “syntax error”.

For example, I created $HOME/.pg_service.conf:
```
[test]
default = true
```

“Default” is not a valid key name, then I got:
```
% PGSERVICE=test psql -d evantest
psql: error: syntax error in service file "/Users/chaol/.pg_service.conf", line 2
```

The error message showed the correct line number, but “syntax error” is confusing. Shouldn’t it be something like
“invalidconfiguration parameter: default”? 

By contrast, If I put a correct key name with a wrong value, the error message is clear, the error message is:
```
% PGSERVICE=test psql -d evantest
psql: error: invalid sslmode value: "disabled"
```

Is that something can be enhanced?

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/