Implementation specific build_exception

[bjfish]

The current build_exception relies on the Exception implementation containing a instance variable mesg: https://github.com/ruby/ruby/blob/c8661de0143c1d434dd9f68910ae304c59f8576b/ext/psych/psych_to_ruby.c#L9-L16

Should this use rb_exc_new_str() (which would also call initialize)?

cc: @eregon