Provide clearer error messages when no driver is configured in the offline runner

[andrewpillar]

Version djinn devel +b2456a5 Sat Apr 24 16:50:25 2021 +0100 linux/amd64

Environment --

Describe the bug
Segfault happens when you run the offline runner with an empty driver.conf file,

[main][~/code/djinn-ci/djinn]$ cat ~/.config/djinn/driver.conf
[main][~/code/djinn-ci/djinn]$ djinn
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0xa0f214]

goroutine 1 [running]:
main.main()
    /home/andrew/code/djinn-ci/djinn/cmd/djinn/main.go:202 +0x11b4

this should instead just throw up an error complaining about no drivers having been configured.

Reproduction steps
Create an empty driver.conf file and run djinn.