Improve the error message for an invalid commit SHA
Improve the error message for an invalid commit SHA

Fixes #54.

fizzy123/homu,coupa/homu,tempbottle/homu,frewsxcv/homu,mvines/homu,karen-irc/homu,servo/homu,frewsxcv/homu,fizzy123/homu,barosl/homu,servo/homu,barosl/homu,karen-irc/homu,mvines/homu,coldnight/homu,coupa/homu,acmcarther/homu,acmcarther/homu,tempbottle/homu,coldnight/homu