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.cpp b/init/subcontext.cpp
index a13f0c7..00f91d8 100644
--- a/init/subcontext.cpp
+++ b/init/subcontext.cpp
@@ -151,15 +151,15 @@
 void SubcontextProcess::ExpandArgs(const SubcontextCommand::ExpandArgsCommand& expand_args_command,
                                    SubcontextReply* reply) const {
     for (const auto& arg : expand_args_command.args()) {
-        auto expanded_prop = std::string{};
-        if (!expand_props(arg, &expanded_prop)) {
+        auto expanded_arg = ExpandProps(arg);
+        if (!expanded_arg) {
             auto* failure = reply->mutable_failure();
-            failure->set_error_string("Failed to expand '" + arg + "'");
+            failure->set_error_string(expanded_arg.error().message());
             failure->set_error_errno(0);
             return;
         } else {
             auto* expand_args_reply = reply->mutable_expand_args_reply();
-            expand_args_reply->add_expanded_args(expanded_prop);
+            expand_args_reply->add_expanded_args(*expanded_arg);
         }
     }
 }