AAPT2: Add missing error statement

Change-Id: I955ba6f151d68db02c3dc16d121994fb553dcea8
diff --git a/tools/aapt2/link/ReferenceLinker.cpp b/tools/aapt2/link/ReferenceLinker.cpp
index fe886de..6aa9c0e 100644
--- a/tools/aapt2/link/ReferenceLinker.cpp
+++ b/tools/aapt2/link/ReferenceLinker.cpp
@@ -231,6 +231,7 @@
                                                                std::string* outError) {
     const SymbolTable::Symbol* symbol = resolveSymbol(reference, nameMangler, symbols);
     if (!symbol) {
+        if (outError) *outError = "not found";
         return {};
     }