Dump git command error instead of silent fail

Bug: 747349
Change-Id: Ied3abb6f93a391bf6581275efbd96b3763413de9
Reviewed-on: https://chromium-review.googlesource.com/976540Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Achuith Bhandarkar <achuith@chromium.org>
Commit-Queue: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#546109}

build/util/lastchange.py

@@ -8,6 +8,7 @@ lastchange.py -- Chromium revision fetching utility.
 """
 
 import re
+import logging
 import optparse
 import os
 import subprocess
@@ -42,7 +43,8 @@ def RunGitCommand(directory, command):
                             cwd=directory,
                             shell=(sys.platform=='win32'))
     return proc
-  except OSError:
+  except OSError as e:
+    logging.error('Command %r failed: %s' % (' '.join(command), e))
     return None
 
 
@@ -174,6 +176,8 @@ def main(argv=None):
                     default='^Change-Id:')
   opts, args = parser.parse_args(argv[1:])
 
+  logging.basicConfig(level=logging.WARNING)
+
   out_file = opts.output
   header = opts.header
   filter=opts.filter