features: add -w for warning and -W for errors

* add val Omd_utils.warn : ?(we=false) -> string -> unit
* add exception Omd_utils.Error of string
* add options -w and -W (omd_main.ml)
* fix: parsing bug when a tag using text/omd is closed on the same line and the contents indicates a code block (this way of writing is wrong anyway, but this fix prevents OMD to suddenly die, and if -w or -W is used, a proper warning or error message is given)

modified:   omd_main.ml
modified:   omd_parser.ml
modified:   omd_parser.mli
modified:   omd_utils.ml
modified:   omd_utils.mli