Line error should include the line num (i.e. index) for easier resolving.

Current line error returns string including line object.

example:
 `"#<SRT::Line:0x007fa5ac8ddf70>, Invalid Time Line formatting, [de vivir en granjas,</i>\r]"`

To make resolving the error easier, it should return the line index:
 `"842, Invalid Time Line formatting, [de vivir en granjas,</i>\r]"`