stringify exceptions to move away from Exception.message

Python 2.6 deprecated the Exception.message field and Python 3 has
dropped it entirely.  Using str() on the exception should get us
all the same info, so switch to that (or implicitly so when using
%s with formatting output).

BUG=chromium:980619
TEST=`cros lint` is clean w/pylint-1.7 again
TEST=`./run_tests` passes

Change-Id: Iedef5c123f7638e08364f8d7256ec0b1cb3f4be0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1829813
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
diff --git a/api/controller/artifacts.py b/api/controller/artifacts.py
index 5d9ae8f..6183fc4 100644
--- a/api/controller/artifacts.py
+++ b/api/controller/artifacts.py
@@ -154,7 +154,7 @@
     # Note that this returns the full path to *multiple* tarballs.
     archives = artifacts.BundleAutotestFiles(chroot, sysroot, output_dir)
   except artifacts.Error as e:
-    cros_build_lib.Die(e.message)
+    cros_build_lib.Die(e)
 
   for archive in archives.values():
     output_proto.artifacts.add().path = archive