how to make this function nothrow?

[Jack via Digitalmars-d-learn]

I have to make my function nothrow because the function that 
calls it (not written by me) is nothrow. So I need to wrap my 
code in a try-catch() but how will I report the error message, if 
the toString() from Throwable isn't nothrow? how do I get out 
this circular dependence?

void f() nothrow
{
  import std.conv : to;

    try
    {
        // do something
    }
    catch(Throwable th)
    {
        auto err = th.toString;
    }
}

I can't use err variable, it result in error:

function object.Throwable.toString is not nothrow

obviously, insert a try-catch() within catch() is a circular 
dependence and doesn't solve the problem either (even if it, I 
think it would be quite ugly)