Add error message when GPTQ-for-LLaMa import fails

[matthew-mcallister]

I'm of the opinion that users should not have to Google random error messages for 30 minutes when a program crashes, so I decided to do something constructive and add helpful instructions for when loading a GPTQ model fails.

EDIT: FWIW, a super helpful improvement would be to check in GPTQ-for-LLaMa as a git submodule; that way git submodule update --init would automatically download the external dependency without having to follow an extra documentation page in the first place.