[unused vars] Improve error messages in templates

This adds the location of the invocation that encountered the
unused var to the stack of 'whence it was called' lines.

Change-Id: I3185372750d15171aa7650d1bf4618d3d6e076eb

src/gn/scope.cc

diff --git a/src/gn/scope.cc b/src/gn/scope.cc
index 24888cf..d07426a 100644
--- a/src/gn/scope.cc
+++ b/src/gn/scope.cc
@@ -259,6 +259,17 @@
           "You set the variable \"" + std::string(pair.first) +
           "\" here and it was unused before it went\nout of scope.";
 
+
+      // Gather the template invocations that led up to this scope.
+      auto entries = GetTemplateInvocationEntries();
+      if (entries.size() != 0) {
+
+        help.append("\n\nVia these template invocations:\n");
+        for (const auto& entry : entries) {
+          help.append("  " + entry.Describe() + "\n");
+        }
+      }
+
       const BinaryOpNode* binary = pair.second.value.origin()->AsBinaryOp();
       if (binary && binary->op().type() == Token::EQUAL) {
         // Make a nicer error message for normal var sets.

src/gn/template.cc

diff --git a/src/gn/template.cc b/src/gn/template.cc
index deafa33..d2c6fc6 100644
--- a/src/gn/template.cc
+++ b/src/gn/template.cc
@@ -116,13 +116,21 @@
   invoker_value = template_scope.GetMutableValue(variables::kInvoker,
                                                  Scope::SEARCH_NESTED, false);
   if (invoker_value && invoker_value->type() == Value::SCOPE) {
-    if (!invoker_value->scope_value()->CheckForUnusedVars(err))
+    if (!invoker_value->scope_value()->CheckForUnusedVars(err)) {
+      // If there was an error, append the caller location so the error message
+      // displays a stack trace of how it got here.
+      err->AppendSubErr(Err(invocation, "whence it was called."));
       return Value();
+    }
   }
 
   // Check for unused variables in the template itself.
-  if (!template_scope.CheckForUnusedVars(err))
+  if (!template_scope.CheckForUnusedVars(err)) {
+    // If there was an error, append the caller location so the error message
+    // displays a stack trace of how it got here.
+    err->AppendSubErr(Err(invocation, "whence it was called."));
     return Value();
+  }
 
   return result;
 }