Hopefully fixes NixOS#3278.

(cherry picked from commit 1ab8d6a)
Signed-off-by: Domen Kožar <domen@dev.si>

When encountering an unsupported protocol, there's no need to retry.
Chances are, it won't suddenly be supported between retry attempts;
error instead. Otherwise, you see something like the following:

    $ nix-env -i -f git://git@github.com/foo/bar
    warning: unable to download 'git://git@github.com/foo/bar': Unsupported protocol (1); retrying in 335 ms
    warning: unable to download 'git://git@github.com/foo/bar': Unsupported protocol (1); retrying in 604 ms
    warning: unable to download 'git://git@github.com/foo/bar': Unsupported protocol (1); retrying in 1340 ms
    warning: unable to download 'git://git@github.com/foo/bar': Unsupported protocol (1); retrying in 2685 ms

With this change, you now see:

    $ nix-env -i -f git://git@github.com/foo/bar
    error: unable to download 'git://git@github.com/foo/bar': Unsupported protocol (1)

(cherry picked from commit c976cb0)
Signed-off-by: Domen Kožar <domen@dev.si>

Includes the expression of the condition in the assertion message if
the assertion failed, making assertions much easier to debug. eg.

    error: assertion (withPython -> (python2Packages != null)) failed at pkgs/tools/security/nmap/default.nix:11:1

(cherry picked from commit 307bcb9)
Signed-off-by: Domen Kožar <domen@dev.si>

(cherry picked from commit 12556e5)
Signed-off-by: Domen Kožar <domen@dev.si>