Fix error handling in case of unicode decode error.

Commit 6fc394f9 added some diagnostics, but it didn't quite work out the
way it was planned.

Bug: 1210746
Change-Id: Ia4f30606e87d3f8b1362441bd74bb7ed0f5ca886
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2928771
Auto-Submit: Greg Thompson <grt@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Dirk Pranke <dpranke@google.com>

presubmit_support.py

@@ -1006,7 +1006,7 @@ class AffectedFile(object):
             self.AbsoluteLocalPath(), 'rU').splitlines()
       except IOError:
         pass  # File not found?  That's fine; maybe it was deleted.
-      except UnicodeDecodeError:
+      except UnicodeDecodeError as e:
         # log the filename since we're probably trying to read a binary
         # file, and shouldn't be.
         print('Error reading %s: %s' % (self.AbsoluteLocalPath(), e))