[utils] Improve swift_build_support.shell failure messages.

- This improves the error messages when commands fail (or don't exist) to show
   a one-line summary of the issue instead of the Python backtrace, and matches
   what was being done by the matching function in `SwiftBuildSupport`.

utils/swift_build_support/tests/test_tar.py

@@ -51,7 +51,7 @@ class TarTestCase(unittest.TestCase):
                          expect.format(dest=destination, source=source))
 
     def test_tar_nonexistent_file_raises(self):
-        with self.assertRaises(subprocess.CalledProcessError):
+        with self.assertRaises(SystemExit):
             tar(source='/path/to/something/that/surely/doesnt/exist',
                 destination='/another/path/that/shouldnt/exist')