made error massage more meaningful and added bitWidth and byteSize

please comment
I think this is the wrong approach. Ultimately, the knowledge of the pointer size is of backend reponsability. The right approach to me seems to add a method to the Backend interface, and implement it in libd-llvm using:
http://llvm.org/docs/doxygen/html/group__LLVMCTarget.html#gab6ea442091017ffe1677f0720cc5aa7a

Then it can be used here.

BTW, do D support 12bits bytes ? I don't think it is part of the standard.