show backtrace when mruby dies on unhandled exception

Fix stack underflow in scope_body() caused by 18dd60c1

delete needless output in String#[]=

Commit #18dd60c1 causes side effects.
This patch fixes stack underflow in scope_body().

fix when mruby parse embedded rd document, mruby throw a syntax error.

Add Test for String#bytes

fix: broken variable reference, use 'next' statement

fix mruby exit status with mrb file.

 Because the tree is gone pop() even if it is empty.