Set default checkstyle severity to error

Author: sbesson

config/checkstyle/checkstyle.xml

@@ -20,7 +20,7 @@
 
   <property name="charset" value="UTF-8"/>
 
-  <property name="severity" value="${org.checkstyle.google.severity}" default="warning"/>
+  <property name="severity" value="${org.checkstyle.google.severity}" default="error"/>
 
   <property name="fileExtensions" value="java, properties, xml"/>
   <!-- Excludes all 'module-info.java' files              -->