Log IOExceptions on update of project configuration

Without logging these exceptions it's hard to guess why the update of
the project configuration is failing.

Bug: issue 3342
Change-Id: I587e2bbee50f20833f9b820f31e5a12a378f59fb
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

gerrit-server/src/main/java/com/google/gerrit/server/project/PutConfig.java

@@ -191,6 +191,8 @@ public class PutConfig implements RestModifyView<ProjectResource, Input> {
           throw new ResourceConflictException("Cannot update " + projectName
               + ": " + e.getCause().getMessage());
         } else {
+          log.warn(String.format("Failed to update config of project %s.",
+              projectName), e);
           throw new ResourceConflictException("Cannot update " + projectName);
         }
       }