[MINOR] config: improve error reporting in global section

[MINOR] config: improve error reporting in global section

Try not to immediately exit on non-fatal errors while parsing the
global section, so that the user has a chance to get most of the
errors at once, which is quite convenient especially during config
checks with the -c argument. Some other errors such as unresolved
server names also don't make the parser exit too early.
(cherry picked from commit 058e9074865abd23912127d8847cccb6b5f4eb8b)