check `mrb_open` return value
rewrite stripping

Previous version ignored some errors, and didn't free memory and close files.
Now no memory will be dynamically allocated to simplify error handling.

This commit also fixes a wrong check:

  files[i] = fopen(argv[i], "wb");
  if (!ireps[i]) {

Improve error messages a bit and add missing newline to one.
update expected error message in bintest
coding style adjustments
initialize `args` in `parse_args`
Merge pull request #2457 from cremno/mruby-strip-rewrite

mruby-bin-strip rewrite