hooks: Make the error when upstream not found more actionable

Currently a stack trace is spit when a change is tried to be uploaded
before running `repo start` on a project. This change replaces that
stack trace with a terser error and instructions to run repo start.

Bug: None
Test: Tried to upload this before calling repo start. Complained.

Change-Id: I01578041fe8fab954c53e8d66750ca6160c62f33