Ignore non-VTS annotations

Currently used only in hidl-gen unit tests.
It'd still make hidl-gen VTS backend more robust.
Yet, empty callflow {} block is emitted to its output .vts file
for human to detect such problems if they miss the stderr msg.

Test: mma
Bug: 34993294
Change-Id: I2dc812d326cf645853d9bf4713df74e3cb18aa30
diff --git a/Interface.cpp b/Interface.cpp
index 174e544..564a534 100644
--- a/Interface.cpp
+++ b/Interface.cpp
@@ -741,10 +741,10 @@
                     }
                 }
             } else {
-                std::cerr << "Invalid annotation '"
+                std::cerr << "Unrecognized annotation '"
                           << name << "' for method: " << method->name()
-                          << ". Should be one of: entry, exit, callflow. \n";
-                return UNKNOWN_ERROR;
+                          << ". A VTS annotation should be one of: "
+                          << "entry, exit, callflow. \n";
             }
             out.unindent();
             out << "}\n";