Don't use exceptions as normal control flow

I think it's generally considered bad practice to use exceptions for normal control flow.

pypocketfft.cc

@@ -152,14 +152,11 @@ template<typename T> py::array sym_rfftn_internal(const py::array &in,
 py::array fftn(const py::array &a, py::object axes, int inorm, bool inplace,
   size_t nthreads)
   {
-  try {
-    return xfftn(a, axes, inorm, inplace, true, nthreads);
-    }
-  catch (runtime_error &)
-    {
+    if (a.dtype().kind() == 'c')
+      return xfftn(a, axes, inorm, inplace, true, nthreads);
+
     if (inplace) throw runtime_error("cannot do this operation in-place");
     DISPATCH(a, f64, f32, flong, sym_rfftn_internal, (a, axes, inorm, nthreads))
-    }
   }
 
 py::array ifftn(const py::array &a, py::object axes, int inorm,