Output message for OutputProcessor errors

instead of just an empty string. This should happen almost never, but
hopefully this will prompt people to report bugs if it does happen
instead of hiding them.

lib/brakeman/processors/output_processor.rb

@@ -10,7 +10,7 @@ class Brakeman::OutputProcessor < Ruby2Ruby
 
   #Copies +exp+ and then formats it.
   def format exp
-    process(exp.deep_clone) || ""
+    process(exp.deep_clone) || "[Format Error]"
   end
 
   alias process_safely format

test/tests/test_output_processor.rb

@@ -6,19 +6,19 @@ class OutputProcessorTests < Test::Unit::TestCase
   end
 
   def test_output_nil
-    assert_output "", nil
+    assert_output "[Format Error]", nil
   end
 
   def test_output_empty_sexp
-    assert_output "", Sexp.new
+    assert_output "[Format Error]", Sexp.new
   end
 
   def test_output_missing_node_type
-    assert_output "", Sexp.new(Sexp.new(:str, 'x'))
+    assert_output "[Format Error]", Sexp.new(Sexp.new(:str, 'x'))
   end
 
   def test_output_bad_node_type
-    assert_output "", Sexp.new(:bad_node_type)
+    assert_output "[Format Error]", Sexp.new(:bad_node_type)
   end
 
   def test_output_local_variable