As it was, any error in a weighted mapping would return a very unhelpfull error message.

e.g. `nats-server: mappingtest.cfg:38:39: interface conversion: interface {} is []interface {}, not string`

This was because the line `err := &configErr{tk, fmt.Sprintf("Error adding mapping for %q to %q : %v", subj, v.(string), err)}` would panic on the `v.(string)` since in weighted mapping that interface{} is actually a map[string]interface{} (since there's can be more than one mapping in weighted mappings).

Now returns the actual error:

e.g. `nats-server: mappingtest.cfg:40:3: Error adding mapping for "bla" : invalid mapping destination: wildcard index out of range in {{wildcard(1)}}`

2: improves subject transform checking and catches if the destination is using a mapping function and there are no partial wildcards in the source.
Signed-off-by: Jean-Noël Moyne <jnmoyne@gmail.com>

Signed-off-by: Jean-Noël Moyne <jnmoyne@gmail.com>