Subprocess error handling

[berdario]

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
• 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.