Fix crash during presubmit checking.

If a presubmit happened to generate a crash, the logic we have
for reporting the presubmit check times would call `six.reraise()`
incorrectly, leading to a second crash.

This CL fixes the invocation of the second crash, and also adds
an error message to help users tripping over one possible source
of the first crash (reading a binary file as text).

Bug: 1210746
Change-Id: Ic46f38901b6acf2055b3feb7272dc751dc69037c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2921322
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@google.com>

presubmit_support.py

@@ -1006,6 +1006,12 @@ class AffectedFile(object):
             self.AbsoluteLocalPath(), 'rU').splitlines()
       except IOError:
         pass  # File not found?  That's fine; maybe it was deleted.
+      except UnicodeDecodeError:
+        # 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))
+        raise
+
     return self._cached_new_contents[:]
 
   def ChangedContents(self, keeplinebreaks=False):
@@ -1674,9 +1680,8 @@ class PresubmitExecuter(object):
       # TODO(crbug.com/953884): replace reraise with native py3:
       #   raise .. from e
       e_type, e_value, e_tb = sys.exc_info()
-      six.reraise(e_type, 'Evaluation of %s failed: %s' % (function_name,
-                                                           e_value),
-                  e_tb)
+      print('Evaluation of %s failed: %s' % (function_name, e_value))
+      six.reraise(e_type, e_value, e_tb)
 
     elapsed_time = time_time() - start_time
     if elapsed_time > 10.0:
@@ -1742,7 +1747,7 @@ def DoPresubmitChecks(change,
 
     python_version = 'Python %s' % sys.version_info.major
     if committing:
-      sys.stdout.write('Running %s presubmit commit checks ...\n' % 
+      sys.stdout.write('Running %s presubmit commit checks ...\n' %
                        python_version)
     else:
       sys.stdout.write('Running %s presubmit upload checks ...\n' %