zed auth login: no DISPLAY environment variable specified

[mattnibs]

@philrz ran into this issue when running zed auth login from an ssh shell into a Linux VM.

[philrz]

FWIW, I already knew from the README that it was going to pop up the browser to complete the auth step, so between that and familiarity with DISPLAY's relevance to X window environments, I knew what was going on. But if you can catch that error and show a friendlier message (or actually invoke a CLI-centric auth interface) that might help users that are less familiar.