Linkifier: Don't log stack trace on invalid commentlink match regex

Log only the exception message at WARN level.  If debug mode is
enabled, log the stack trace as before, at DEBUG level.

Change-Id: Ic15c2c59a875d25d978b252ead5f3875993ce88e

gitiles-servlet/src/main/java/com/google/gitiles/Linkifier.java

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/Linkifier.java b/gitiles-servlet/src/main/java/com/google/gitiles/Linkifier.java
index 27aff63..6ef9e05 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/Linkifier.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/Linkifier.java
@@ -79,7 +79,12 @@
       try {
         pattern = Pattern.compile(match);
       } catch (PatternSyntaxException ex) {
-        log.warn("invalid commentlink." + subsection + ".match", ex);
+        String msg = "invalid commentlink." + subsection + ".match";
+        if (log.isDebugEnabled()) {
+          log.debug(msg, ex);
+        } else {
+          log.warn(msg + ": " + ex.getMessage());
+        }
         continue;
       }
       if (Strings.isNullOrEmpty(link)) {