config: replace err with err.message in validate_uri_field

Follows-up previous commits in scope of config validation.

Needed for tarantool#9105

NO_CHANGELOG=<tests change>
NO_DOC=<tests change>

Author: CuriousGeorgiy

src/box/lua/config/instance_config.lua

@@ -196,7 +196,7 @@ local function validate_uri_field(has_login_field, used_to_connect)
             if data:find(',') then
                 w.error('A single URI is expected, not a list of URIs')
             end
-            w.error('Unable to parse an URI: %s', err)
+            w.error('Unable to parse an URI: %s', err.message)
         end
         if uri.login ~= nil and has_login_field then
             w.error("Login must be set via the 'login' option")
@@ -2351,6 +2351,12 @@ return schema.new('instance_config', schema.record({
         }, {
             default = 'new',
         }),
+        box_error_serialize_verbose = schema.enum({
+            'old',
+            'new',
+        }, {
+            default = 'new',
+        }),
     }),
 }, {
     -- This kind of validation cannot be implemented as the