Make `Pkg.build` error message clearer.

An itch I've long wanted to scratch.

Old:

```
WARNING: Images and IJulia had build errors.

 - packages with build errors remain installed in /home/beyer/.julia
 - build a package and all its dependencies with `Pkg.build(pkg)`
 - build a single package by running its `deps/build.jl` script
```

New:

```
WARNING: Images and IJulia had build errors.

 - packages with build errors remain installed in /home/beyer/.julia
 - build the package(s) and all dependencies with `Pkg.build("Images", "IJulia")`
 - build a single package by running its `deps/build.jl` script
```