injector: Repeat injection in case of failure and add proper exception handling

[psrok1]

Is your feature request related to a problem? Please describe.

injector isn't reliable as it interferes with processes already running in the system. We might be out of luck and get a trap being in the wrong place at the wrong time. These situations are usually one-time, so we should just repeat an injection few times until we get satisfying result.

It's especially important in draksetup postinstall step that makes dozen of injector -m readfile and single failure makes profile incomplete. In worst case it may fail at the beginning: #808. In that case, it's difficult to recover for newbie user from that situation as vm-0 is destroyed and we need to manually restore it and make another run.