Invalid error message parsing subject maps

[ColinSullivan1]

Defect

This is minor, but could save certain users some time.

The NATS server is erroneously reporting an interface conversion error when processing configuration files and detecting errors with subject maps (in this case more than 100% allocation with weighted mapping). This is especially confusing when the server is parsing JSON configuration files, as these types of errors can be reported from invalid json.

• Included nats-server -DV output

nats-server: sample.json:6:53: interface conversion: interface {} is []interface {}, not string

• Included a [Minimal, Complete, and Verifiable example] (https://stackoverflow.com/help/mcve)

 $ cat sample.json 
accounts {
  testacct {
    "mappings": {
       "s1": [
          { "destination": "d1.1", "weight": "50%" },
          { "destination": "d1.2", "weight": "51%" } 
       ] 
    }
  }
}

$ nats-server -config sample.json

Versions of nats-server and affected client libraries used:

nats-server: v2.9.17-beta.1

OS/Container environment: N/A

Steps or code to reproduce the issue:

Run the server with the configuration provided above.

Expected result:

An error message indicating the total weight needs to be <= 100

Actual result:

interface conversion: interface {} is []interface {}, not string

Suggestions

I'll see if I can get a test setup later with a PR but in case someone beats me to it, the fix would appear to be replacing string with []interface{}
here:

nats-server/server/opts.go

Line 2563 in b0d98df

err := &configErr{tk, fmt.Sprintf("Error adding mapping for %q to %q : %v", subj, v.(string), err)}

and with and {}interface here:

nats-server/server/opts.go

Line 2575 in b0d98df

err := &configErr{tk, fmt.Sprintf("Error adding mapping for %q to %q : %v", subj, v.(string), err)}

[jnmoyne]

This has now been fixed a while ago by #4202

Currently the mapping example above returns the following error:
Error adding mapping for "s1" : total weight needs to be <= 100