[auth] log errors from luci-auth token invocation.

Motivating example -- seemingly empty output in this build:
https://luci-milo.appspot.com/ui/p/devtools-internal/builders/ci/Auto-roll%20-%20frontend/587/overview
R=vadimsh

Change-Id: I12f1acb1b4852f57057684b26fb153ab74e2da39
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2871721
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Andrii Shyshkalov <tandrii@google.com>
Auto-Submit: Andrii Shyshkalov <tandrii@google.com>

auth.py

@@ -159,5 +159,7 @@ class Authenticator(object):
       return AccessToken(
           token_info['token'],
           datetime.datetime.utcfromtimestamp(token_info['expiry']))
-    except subprocess2.CalledProcessError:
+    except subprocess2.CalledProcessError as e:
+      # subprocess2.CalledProcessError.__str__ nicely formats stdout/stderr.
+      logging.error('luci-auth token failed: %s', e)
       return None