Improve error message for `reference is not a tree`

[parsonsmatt]

I ran into this on the work repo - I did cabal build and got this back:

λ ~/Projects/mercury-web-backend/ avieth/haskell.nix cabal build
fatal: reference is not a tree: 811d89e66bc714ef20f4973e87d999f21c97c4ed

Turns out, we had two entries in cabal.project for that commit:

source-repository-package
  type: git
  location: https://github.com/matthewbauer/amazonka.git
  tag: 811d89e66bc714ef20f4973e87d999f21c97c4ed
  --sha256: sV5aefBrSbz4wiodhE54w85tzScJgSjc1aH+L2520rs=
  subdir: amazonka

source-repository-package
  type: git
  location: https://github.com/MercuryTechnologies/amazonka.git
  tag: 811d89e66bc714ef20f4973e87d999f21c97c4ed
  --sha256: sV5aefBrSbz4wiodhE54w85tzScJgSjc1aH+L2520rs=
  subdir: core

It'd be nice if cabal printed out more information about the source-repository-package it was unpacking when an error like this occurs.