Refine output of `mrb_print_error()`

The following improvements are made according to Ruby's behavior:

- Match location number to index.
- Remove duplicate most recent call output.
- Fix that first call is not output when array (unpacked) backtrace.

### Example

  ```ruby
  def a; raise "error!" end
  def b; a end
  begin
    b
  rescue => e
    e.backtrace if ARGV[0] == "unpack"  # unpack backtrace
    raise e
  end
  ```

#### Before this patch:

  ```
  $ bin/mruby example.rb unpack
  trace (most recent call last):
    [0] example.rb:2:in b
    [1] example.rb:1:in a
  example.rb:1: error! (RuntimeError)
  ```

#### After this patch:

  ```
  $ bin/mruby example.rb unpack
  trace (most recent call last):
    [2] example.rb:4
    [1] example.rb:2:in b
  example.rb:1:in a: error! (RuntimeError)
  ```

0 files changed, 0 insertions, 0 deletions