Subprocess error handling

There're plenty of things that can go wrong during the compilation of protobuf:

- Missing c++ compiler (but with the C and gcc preprocessor installed)
- this [issue](http://code.google.com/p/open-lighting/issues/detail?id=52)
- `javax.tools.ToolProvider/getSystemJavaCompiler` will yield `nil` (even if the OpenJDK is installed and javac is in the PATH... I get this issue on Fedora but not on Ubuntu)

thus, it'd be better to fail fast and report errors when they happen.

lein-protobuf currently doesn't even output at all the stderr of `./configure` and `make`.

The first commit does it, but by printing the commands output when they exit. In the second commit I updated `conch`, but there seems to still be some issues with the output buffering and the change is slightly more invasive.

I chose to avoid printing the stderr right away, and print it only at the end if the command failed, but maybe it'd be better to just print a generic error message after the full output.