odgi_to_rdf misleading error message when 2nd argument is missing

When `odgi_to_rdf` is executed without the 2nd argument, the error message is very cryptive about what is going on.
```
(/usr) [heumos@wave spodgi]$ time python odgi_to_rdf.py --syntax=ttl0k_R64-1-1.odgi 
Usage: odgi_to_rdf.py [OPTIONS] ODGIFILE TTL
Try "odgi_to_rdf.py --help" for help.

Error: Missing argument "TTL".

real    0m0.256s
user    0m0.231s
sys     0m0.025s
```