Better error message when toast does not have permissions to run without sudo

[CodeGradox]

Description
I was going through the guide when I got the error "Unable to pull image" without any details.
Turns out I was just missing sudo...
However, it might be helpful for users who are new to docker, toast or Linux to get a more helpful error message.

Here is the output I got.

$ toast
[INFO] Ready to run 1 task: greet.
[INFO] Running task greet…
[ERROR] Unable to pull image. Details:

I can run toast without sudo on osx so there is no issue there.

System
Arch Linux 5.1.6
Docker 18.09.6-ce
rustc 1.34.2
Toast 0.24.0

[stepchowfun]

Thank you for this bug report! I will take a look this weekend.

[stepchowfun]

Fixed by #248 and shipped in v0.25.0.