Make Pkg.build error message clearer.

[lucasb-eyer]

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

[lucasb-eyer]

I think Travis fails because of an unrelated

/bin/sh: 1: Syntax error: "(" unexpected

[staticfloat]

Yes, I like this change. The travis failure is unrelated.

[ivarne]

I like this too. Scheduling for @JuliaBackports?

[tkelman]

Bump. I can't seem to get Travis to rebuild this one, could you push this again? Maybe rebase for the heck of it, though it's not strictly required since there are no conflicts here.

[lucasb-eyer]

Rebased. I hope I did it right.

[ivarne]

Looks good!

[ivarne]

Backported in 3f29e97