Conditionally log evaluation failure warnings.

Logging was happening unconditionally and that is rather jarring
in certain circumstances, e.g., when ceres is being called
inside a RANSAC loop.

Thanks to Bryan Klingner for reporting this.

Change-Id: I97a0b6c48df51f5f7362eb9d46654712ef044a00

internal/ceres/trust_region_minimizer.cc

diff --git a/internal/ceres/trust_region_minimizer.cc b/internal/ceres/trust_region_minimizer.cc
index 7352ac2..c41b189 100644
--- a/internal/ceres/trust_region_minimizer.cc
+++ b/internal/ceres/trust_region_minimizer.cc
@@ -410,13 +410,15 @@
                                  NULL,
                                  NULL,
                                  NULL)) {
-          LOG(WARNING) << "Step failed to evaluate. "
-                       << "Treating it as a step with infinite cost";
+          LOG_IF(WARNING, is_not_silent)
+              << "Step failed to evaluate. "
+              << "Treating it as a step with infinite cost";
           new_cost = numeric_limits<double>::max();
         }
       } else {
-        LOG(WARNING) << "x_plus_delta = Plus(x, delta) failed. "
-                     << "Treating it as a step with infinite cost";
+        LOG_IF(WARNING, is_not_silent)
+            << "x_plus_delta = Plus(x, delta) failed. "
+            << "Treating it as a step with infinite cost";
       }
 
       if (new_cost < std::numeric_limits<double>::max()) {