`stack ghci` succeeds (with warnings) despite hidden packages

```
$ stack new repro
$ cd repro
$ # Add dependency on package clock in library section of cabal file
$ # In app/Main.hs: Add import statement for `System.Clock`
$ stack ghci
```

What I expected: The command fails.

What actually happened:

```
repro-0.1.0.0: configure
Configuring repro-0.1.0.0...
repro-0.1.0.0: build
Preprocessing library repro-0.1.0.0...
[1 of 1] Compiling Lib ( src/Lib.hs, .stack-work/dist/x86_64-linux/Cabal-1.24.0.0/build/Lib.o )
Preprocessing executable 'repro-exe' for repro-0.1.0.0...

/home/simon/test/repro/app/Main.hs:4:8:
Could not find module ‘System.Clock’
It is a member of the hidden package ‘clock-0.7.2@CEVm9DSU9Qg1Hme0DtWvh6’.
Perhaps you need to add ‘clock’ to the build-depends in your .cabal file.
Use -v to see a list of the files searched for.

-- While building package repro-0.1.0.0 using:
/home/simon/.stack/setup-exe-cache/x86_64-linux/setup-Simple-Cabal-1.24.0.0-ghc-7.10.3 --builddir=.stack-work/dist/x86_64-linux/Cabal-1.24.0.0 build lib:repro exe:repro-exe --ghc-options " -ddump-hi -ddump-to-file"
Process exited with code: ExitFailure 1
Warning: build failed, but optimistically launching GHCi anyway
The following GHC options are incompatible with GHCi and have not been passed to it: -threaded
Using main module: 1. Package `repro' component exe:repro-exe with main-is file: /home/simon/test/repro/app/Main.hs
Configuring GHCi with the following packages: repro
GHCi, version 7.10.3: http://www.haskell.org/ghc/ :? for help
[1 of 1] Compiling Lib ( /home/simon/test/repro/src/Lib.hs, interpreted )
Ok, modules loaded: Lib.
[2 of 2] Compiling Main ( /home/simon/test/repro/app/Main.hs, interpreted )
Ok, modules loaded: Lib, Main.
```

Now, there is a warning, which is good. But if you're using, for example, ghcid, that warning might not be visible to you.

I think it would be better to fail in this case, but maybe I'm overlooking some technicalities of ghci here?