Add multiline description support, more helpful error messages

[felixvd]

I've encountered some cases that don't tell the user what to fix. This catches the exception and localizes the error.

It also supports multi-line comments.

Example:

def myfunc():
    """Text
    
    :param myparam1: text followed by an empty line (will throw an error)
    
    :param myparam2: more text
        with an extra line (will work fine)
    :param myparam3 which has no closing colon (will throw an error)`
    """
    pass