[BUG] config: fix error message when config file is not found

[BUG] config: fix error message when config file is not found

Cameron Simpson reported an annoying case where haproxy simply reports
"Error(s) found in configuration file" when the file is not found or
not readable.

Fortunately the parsing function still returns -1 in case of open
error, so we're able to detect the issue from the caller and report
the corresponding errno message.