DEV: improve error message for invalid setting’s value (PR #13416)

Before this fix we would display this exception:

Discourse::InvalidParameters:
  value

After this fix we will display:

Discourse::InvalidParameters:
  Invalid `x` value for `s3_region`

GitHub

IMO it would read better like:

        raise Discourse::InvalidParameters.new("Invalid value `#{val}` for `#{name}`") unless enum_class(name).valid_value?(val)