* guix/store.scm (&nix-error, &nix-protocol-error): New SRFI-35 condition types. (process-stderr): Raise an error condition upon protocol errors instead of returning to the caller. This allows the connection to be reused for further interactions.
* guix.scm: Move contents to other files. Just aggregate these. * guix/derivations.scm, guix/store.scm: New files.