Better/useful errors for configuration failures

Some of the error messages fluent bit prints when config is invalid are not very useful at all. They do not tell users why their config is invalid. Or, even if they do give you the reason, they won't tell you exactly where in the config file the problem is.

For example, if you use an @INCLUDE statement to import a file which does not exist:

/fluent-bit/etc/fluent-bit.conf
```
@INCLUDE /does-not-exist.conf
```

You get this error:

> Error: Configuration file contains errors. Aborting

Which is entirely useless...