cmd/compile: better error message for failed string conversion

https://play.golang.org/p/8ADwHokqcK5 complains with:
```
prog.go:6:12: cannot use x (type []T) as type []byte in argument to runtime.slicebytetostring
```
The error message should not mention `runtime.slicebytetostring` which is an implementation detail.

(There are probably more error situations that expose runtime calls this way. Should handle them all uniformly.)
