Removing assert(0) and adding an informative compiler exception.

First attempt at a fix for https://github.com/deadalnix/libd/issues/31.

Prints:
```
    p = (cast(ulongptr_t*)(a + (void *).sizeof);
        ^
        test.d:4: error: Matching delimiter not found
test.d line 4
d.exception.CompileException@libd-llvm/libd/src/d/exception.d(20): test.d:4: error: Matching delimiter not found
```