Since BKE_nurb_type_convert will only ever return false if we have less
than 6 points in the curve, we can also be specific in the error
While this change is OK, I think it would be better if this was set by the function it's self.
BKE_nurb_type_convert could take and optional const char **r_err_msg, and assign this in the case of an error.
This means if it ever returns false for another reason - it's less likely a developer doesn't notice the error needs updating elsewhere, or if we want to report the message elsewhere - we don't need to copy/paste messages about.