add explicit unknown token error message

src/Language/SystemVerilog/Parser/Lex.x

@@ -509,6 +509,8 @@ postProcess stack (Token Id_escaped str pos : ts) =
     where
         t' = Token Id_escaped str' pos
         str' = (++ " ") $ init str
+postProcess _ (Token Unknown str pos : _) =
+    throwError $ show pos ++ ": unknown token '" ++ str ++ "'"
 postProcess [] (t : ts) = do
     ts' <- postProcess [] ts
     return $ t : ts'

test/error/double_backtick.sv

@@ -1,4 +1,5 @@
-// pattern: Parse error: unexpected token '`'
+// pattern: unknown token '`'
+// location: double_backtick.sv:4:5
 module top;
     ``
 endmodule

test/error/stray_escaped_vendor_comment.sv

@@ -1,4 +1,5 @@
-// pattern: Parse error
+// pattern: unknown token '`'
+// location: stray_escaped_vendor_comment.sv:4:6
 module top;
     /``* some awful garbage *``/
 endmodule