re2: Wrap unguarded uses of LOG(ERROR), testing options.log_errors().

Most uses were already guarded, but a few were not, and were generating
unwanted stderr output.

R=rsc
CC=re2-dev
http://codereview.appspot.com/6843067

CONTRIBUTORS

diff --git a/CONTRIBUTORS b/CONTRIBUTORS
index ac64332..7b44e04 100644
--- a/CONTRIBUTORS
+++ b/CONTRIBUTORS
@@ -27,6 +27,7 @@
 # Please keep the list sorted.
 
 Dominic Battré <battre@chromium.org>
+John Millikin <jmillikin@gmail.com>
 Rob Pike <r@google.com>
 Russ Cox <rsc@swtch.com>
 Sanjay Ghemawat <sanjay@google.com>