move src/error.h to include/mruby/error.h

Just a proposal since this cause header including error in many [iij](https://github.com/iij) mrbgems(io, socket, etc...).
With this change following in *mrbgem.rake* can be removed:
```ruby
spec.cc.include_paths << "#{build.root}/src"
```

Is it possible to detect mruby version in *rake* files, so that mrbgems that gets error with this change can have workaround?