Improve error message when an unknown SWIG directive is used

This previously gave the cryptic "Error: Syntax error in input(1).", but
now gives "Error: Unknown directive '%foo'."

Examples/test-suite/errors/pp_unknowndirective.i

@@ -0,0 +1,7 @@
+%module xxx
+
+/* This used to give the rather cryptic "Syntax error in input(1)." prior to
+ * SWIG 3.0.4.  This testcase checks that the improved message is actually
+ * issued.
+ */
+%remane("typo") tyop;

Examples/test-suite/errors/pp_unknowndirective.stderr

@@ -0,0 +1 @@
+c_unknowndirective.i:7: Error: Unknown directive '%remane'.