fix identifier error message of partial with unvalid charter

The partial should start with an underscore not with a underscore or letter.
It  possibile render an alias partial inside a view using the <%= render file: 'name/no_start_with_undescore' %>
but it is not a partial.

before this PR IDENTIFIER_ERROR_MESSAGE could lead to misunderstand the convention of partial name.
Added OPTION_AS_ERROR_MESSAGE for unvalid charter in as option.