Improve error message for `reference is not a tree`

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.