Improve error reporting

[peterbecker]

At the moment errors will be reported one at a time. It would be nicer if the configuration parser could validate all options and then produce a report on all missing or erroneous values at once.

[peterbecker]

Also: missing elements on sub-interfaces will not indicate the full path, only the local name of the property. For example: if a JDBC configuration object has a url() method and is used as nested config via database() method, then we currently report "url" as missing, but it should be "database.url".

[peterbecker]

I split the original issue into #24, this ticket's name was very generic and I already added the other, smaller problem in the comment. Easier this way around - the ticket shall now refer only to the comment.