Yield nice errors in pkg-config isn't available

[lovesegfault]

If you try to use this in a system where pkg-config is not installed (e.g. a fresh Ubuntu install) you get a rather cryptic panic

error: failed to run custom build command for `musync v0.1.0 (file:///home/bemeurer/src/musync)`
process didn't exit successfully: `/home/bemeurer/src/musync/target/release/build/musync-9f053b254a4fda51/build-script-build` (exit code: 101)
--- stderr
thread 'main' panicked at 'Failed to detect libmad: Command { command: "\"pkg-config\" \"--libs\" \"--cflags\" \"mad\"", cause: Error { repr: Os { code: 2, message: "No such file or directory" } } }', /checkout/src/libcore/result.rs:906:4

Shouldn't this give a more readable message simply saying pkg-config isn't installed?