Remove location info from Exception#inspect

Because location info (file name and line number) is kept in the backtrace,
it should not be kept in the result of `inspect` (and the exception object
itself), I think.

### Example

  ```ruby
  # example.rb
  begin
    raise "err"
  rescue => e
    p e
  end
  ```

#### Before this patch:

  ```
  $ bin/mruby example.rb
  example.rb:2: err (RuntimeError)
  ```

#### After this patch:

  ```
  $ bin/mruby example.rb
  err (RuntimeError)
  ```

include/mruby/class.h

@@ -90,6 +90,7 @@ mrb_value mrb_instance_new(mrb_state *mrb, mrb_value cv);
 void mrb_class_name_class(mrb_state*, struct RClass*, struct RClass*, mrb_sym);
 mrb_bool mrb_const_name_p(mrb_state*, const char*, mrb_int);
 mrb_value mrb_class_find_path(mrb_state*, struct RClass*);
+mrb_value mrb_mod_to_s(mrb_state*, mrb_value);
 void mrb_gc_mark_mt(mrb_state*, struct RClass*);
 size_t mrb_gc_mark_mt_size(mrb_state*, struct RClass*);
 void mrb_gc_free_mt(mrb_state*, struct RClass*);