Improve "Unable to get forkpoint" error message

[Alberto Leal]

This error message isn't very useful, and the user will get stuck not knowing what the next steps are:

sentry (jferg/scim-doc-6):$ git chain rebase
error: Unable to get forkpoint of jferg/scim-doc-2 and jferg/scim-doc-3

It'll be helpful to provide some suggestions.