Use `Exception#cause` when raising exceptions thrown in subproject execution

[michaeljbishop]

The super-project will catch Exceptions that the sub-project raises as they are carried across the pipe, but they shoudl be reported in the cause field of the proxy exception raised in the super-project execution.

[michaeljbishop]

This is a little tricky because we want to make sure that if an exception is thrown in a sub-sub-sub-sub-project, it's stack trace is propagated and output all the way to the top. At the moment, we don't propagate the cause backtrace, just the main exception.