ART: Run ExceptionDescribe on error

When a classloader cannot be printed, print the exception.

Bug: 109724219
Test: m test-art-host
Change-Id: I32fbd241cb247a0e4cbe8f13a6120a8aa736995a
diff --git a/runtime/jni/java_vm_ext.cc b/runtime/jni/java_vm_ext.cc
index 8fe68bd..44679a5 100644
--- a/runtime/jni/java_vm_ext.cc
+++ b/runtime/jni/java_vm_ext.cc
@@ -912,7 +912,11 @@
               return utf.c_str();
             }
           }
-          env->ExceptionClear();
+          if (env->ExceptionCheck()) {
+            // We can't do much better logging, really. So leave it with a Describe.
+            env->ExceptionDescribe();
+            env->ExceptionClear();
+          }
           return "(Error calling toString)";
         }
         return "null";