When reporting an error for a construct which hasn't been terminated

when the end of the file is reached, report it at the start line rather
than "EOF" as then tools like editors and IDEs will take you to a
generally more useful place for fixing the problem.

Examples/test-suite/errors/pp_missing_endoffile.i

@@ -0,0 +1,7 @@
+%module xxx
+/* %beginfile and %endoffile are internal directives inserted when %include is
+ * used.  Users should never use them directly, but test coverage for this
+ * error message still seems useful to have.
+ */
+%includefile "dummy.i" %beginfile
+