init: don't log in expand_props directly

It's better to pass the error message to the caller to determine how
best to print the error.

Test: build
Change-Id: Id8857c459df2f26c031650166609608d20e4d051
diff --git a/init/subcontext_test.cpp b/init/subcontext_test.cpp
index e120a62..ae89c38 100644
--- a/init/subcontext_test.cpp
+++ b/init/subcontext_test.cpp
@@ -166,7 +166,8 @@
         };
         auto result = subcontext.ExpandArgs(args);
         ASSERT_FALSE(result);
-        EXPECT_EQ("Failed to expand '" + args[1] + "'", result.error().message());
+        EXPECT_EQ("unexpected end of string in '" + args[1] + "', looking for }",
+                  result.error().message());
     });
 }