store: Rename '&nix-error' to '&store-error'.

store: Rename '&nix-error' to '&store-error'.

* guix/store.scm (&nix-error): Rename to...
(&store-error): ... this, and adjust users.
(&nix-connection-error): Rename to...
(&store-connection-error): ... this, and adjust users.
(&nix-protocol-error): Rename to...
(&store-protocol-error): ... this, adjust users.
(&nix-error, &nix-connection-error, &nix-protocol-error): Define these
condition types and their getters as deprecrated aliases.
* build-aux/run-system-tests.scm, guix/derivations.scm,
guix/grafts.scm, guix/scripts/challenge.scm,
guix/scripts/graph.scm, guix/scripts/lint.scm,
guix/scripts/offload.scm, guix/serialization.scm,
guix/ssh.scm, guix/tests.scm, guix/ui.scm,
tests/derivations.scm, tests/gexp.scm, tests/guix-daemon.sh,
tests/packages.scm, tests/store.scm, doc/guix.texi: Adjust to use the
new names.