Added constructor to RangeError which takes an index and length.

This should make the error messages for RangeError clearer. For now,
it'll improve non-druntime code which uses RangeError, but we should
probably make it so that arrays use the new overload as well.

This relates to [phobos pull# 1196](https://github.com/D-Programming-Language/phobos/pull/1196).