User friendly error message when option is not defined

[domenkozar]

Currently, when one does

services.postgresql.enable = true;

it fails with:

user-thrown exception: Not defined.

Due to postgresql.packages not specified, I think we could at least display what option is missing.

[edolstra]

Closing as a special case of issue #15. Note that the stack trace actually does show the name of the undefined option ("while evaluating the option `services.postgresql.package': ...") but it's obscured in the stack trace.

[domenkozar]

See 4278f77