Add -Werror.

Also, fixed several warnings (including one bug in generateCppImpl.cpp).

Test: compiles

Bug: 32559427

Change-Id: I78375b9052c0ad447ebac638258f25bbf562712f
diff --git a/generateCppImpl.cpp b/generateCppImpl.cpp
index 3c2d9b7..1ca899a 100644
--- a/generateCppImpl.cpp
+++ b/generateCppImpl.cpp
@@ -234,6 +234,10 @@
         return generateStubImplMethod(out, baseName, method);
     });
 
+    if (err != OK) {
+        return err;
+    }
+
     out << ifaceName
         << "* ";
     generateFetchSymbol(out, ifaceName);