Report errors when running top-level build

When running the top-level rake tasks, no indication is given if one of the underlying project tasks fails.

This is particularly confusing for new contributors when checking out rails and running rake for the first time. Unless you watch the output like a hawk or scroll back you can easily miss failures and think all the tests have passed.

This simple patch modifies the rakefile to record errors as it runs the tasks. If any fail, the top-level task also fails, listing the failed projects.