Improve error message when attempting to migrate code that depends on third-party unmigrated code

[jathak]

The migrator will usually error when dealing with code that depends on unmigrated external code through more than one entrypoint. This makes sense, since there's often no good way to migrate this code, but the error message is not useful. We should advise users in this case to either consolidate their imports of that dependency to a single entrypoint, or to just wait for the upstream code to migrate before migrating themselves.