[gdb/python] Use gdbpy_handle_gdb_exception in valpy_assign_core

In valpy_assign_core we have:
...
  catch (const gdb_exception &except)
    {
      gdbpy_convert_exception (except);
      return false;
     }
...

Use instead:
...
  catch (const gdb_exception &except)
    {
      return gdbpy_handle_gdb_exception (false, except);
    }
...

No functional changes.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>

gdb/python/py-value.c

@@ -921,8 +921,7 @@ valpy_assign_core (value_object *self, struct value *new_value)
     }
   catch (const gdb_exception &except)
     {
-      gdbpy_convert_exception (except);
+      return gdbpy_handle_gdb_exception (false, except);
-      return false;
     }
 
   return true;