When running a main method, if the user inputs ctrl+c then the `run`
task will exit but the main method is not interrupted so it continues
running even once sbt has returned to the shell. If the main method is a
webserver, this prevents run from ever starting again on a fixed port.
To fix this, we can modify the waitForTry method to stop the job if an
exception is thrown (ctrl+c leads to an interrupted exception being
thrown by waitFor).

I rework the BackgroundJobService so that the default implementation of
waitForTry is now usable and no longer needs to be overridden. The side
effect of this change is that waitFor may now throw an exception. Within
sbt, waitFor was only used in one place and I reworked it to use
waitForTry instead. This could theoretically break a downstream user who
relied on waitFor not throwing an exception but I suspect that there
aren't many users of this api, if any at all.

When a user calls sbt exit and there is an active background job, sbt
may not exit cleanly. This was primarily because the
background job service shutdown method depended on the
StandardMain.executionContext which was closed before the background job
service was shutdown. This was fixable by reordering the resource
closing in StandardMain.runManaged.

Rather than putting the background job temporary files in whatever
java.io.tmpdir points to, this commit moves the files into a
subdirectory of target in the project root directory.

To make the directory configurable via settings, I had to move the
declaration of the bgJobService setting later in the project
initialization process. I don't think this should matter because
background jobs shouldn't be created until after the project has loaded
all of its settings..