kunit: tool: fix invalid attribute access on build failure

On a recent failed run we got the following error:

Traceback (most recent call last):
  File "./tools/testing/kunit/kunit.py", line 123, in <module>
    main(sys.argv[1:], kunit_kernel.LinuxSourceTree())
  File "./tools/testing/kunit/kunit.py", line 116, in main
    result = run_tests(linux, request)
  File "./tools/testing/kunit/kunit.py", line 39, in run_tests
    build_result = linux.build_um_kernel()
  File "/home/prow/go/src/github.com/kunit-review.googlesource.com/linux/tools/testing/kunit/kunit_kernel.py", line 146, in build_um_kernel
    return BuildResult(BuildStatus.FAILURE, e.message)
AttributeError: 'BuildError' object has no attribute 'message'

This fixes the invalid attribute access.

Google-Bug-Id: 131438303
Signed-off-by: Brendan Higgins <brendanhiggins@google.com>
Change-Id: I09b0d493250d686989cbf726dc3e9a4805b7ef94
diff --git a/tools/testing/kunit/kunit_kernel.py b/tools/testing/kunit/kunit_kernel.py
index bd81c82..4b4cffb 100644
--- a/tools/testing/kunit/kunit_kernel.py
+++ b/tools/testing/kunit/kunit_kernel.py
@@ -143,7 +143,7 @@
 			self._ops.make()
 		except (ConfigError, BuildError) as e:
 			logging.error(e)
-			return BuildResult(BuildStatus.FAILURE, e.message)
+			return BuildResult(BuildStatus.FAILURE, str(e))
 		used_kconfig = kunit_config.Kconfig()
 		used_kconfig.read_from_file(KCONFIG_PATH)
 		try: