Improve error message when --json flag is specified (#933)

## Changes
Improve error message when --json input is provided

## Tests
```
cli % databricks model-registry create-model mymodel --json @./input.json              
Error: when --json flag is specified, no positional arguments are required. Provide NAME in your JSON input
```

.codegen/service.go.tmpl

@@ -141,12 +141,20 @@ func new{{.PascalName}}() *cobra.Command {
 	cmd.Annotations = make(map[string]string)
 	{{if $hasRequiredArgs }}
 	cmd.Args = func(cmd *cobra.Command, args []string) error {
-		check := cobra.ExactArgs({{len .Request.RequiredFields}})
 		{{- if and .CanUseJson .Request.HasRequiredRequestBodyFields }}
 		if cmd.Flags().Changed("json") {
-			check = cobra.ExactArgs({{len .Request.RequiredPathFields}})
+			err := cobra.ExactArgs({{len .Request.RequiredPathFields}})(cmd, args)
+			if err != nil {
+			{{- if eq 0 (len .Request.RequiredPathFields) }}
+				return fmt.Errorf("when --json flag is specified, no positional arguments are required. Provide{{- range $index, $field := .Request.RequiredFields}}{{if $index}},{{end}} '{{$field.Name}}'{{end}} in your JSON input")
+			{{- else }}
+				return fmt.Errorf("when --json flag is specified, provide only{{- range $index, $field := .Request.RequiredPathFields}}{{if $index}},{{end}} {{$field.ConstantName}}{{end}} as positional arguments. Provide{{- range $index, $field := .Request.RequiredRequestBodyFields}}{{if $index}},{{end}} '{{$field.Name}}'{{end}} in your JSON input")
+			{{- end }}
+			}
+			return nil
 		}
 		{{- end }}
+		check := cobra.ExactArgs({{len .Request.RequiredFields}})
 		return check(cmd, args)
 	}
 	{{end}}