Reduce log spam in covariance_impl.cc.

Make the rank deficiency warning in the covariance module a VLOG(1) instead of
LOG(ERROR) since this is a commonly occuring case.

Change-Id: Id46eba6dcf9afbaf0554c2d6f230b2af7a1fbdea

internal/ceres/covariance_impl.cc

diff --git a/internal/ceres/covariance_impl.cc b/internal/ceres/covariance_impl.cc
index 1f86707..65fe7c6 100644
--- a/internal/ceres/covariance_impl.cc
+++ b/internal/ceres/covariance_impl.cc
@@ -648,9 +648,9 @@
   }
 
   if (rank < cholmod_jacobian.ncol) {
-    LOG(ERROR) << "Jacobian matrix is rank deficient. "
-               << "Number of columns: " << cholmod_jacobian.ncol
-               << " rank: " << rank;
+    LOG(WARNING) << "Jacobian matrix is rank deficient. "
+                 << "Number of columns: " << cholmod_jacobian.ncol
+                 << " rank: " << rank;
     free(permutation);
     cholmod_l_free_sparse(&R, &cc);
     cholmod_l_finish(&cc);