Improve NixOS evaluation error messages

[edolstra]

If a NixOS option has a value that doesn't match the type of the option, you get a giant stack trace that is pretty hard to read and obscures the actual error message in a trace (e.g. "trace: x is a string ...").

Instead we should turn off strack traces, and the option evaluation code should throw an error message like

error: option ‘services.bla.config’ must have type string but has a value of type integer in module ‘/foo/bar/configuration.nix’

[edolstra]

Likewise for undefined options (#148, #180).