ErrorMessageFormatter

The em string interpolator works like the i string interpolator, but marks nonsensical errors using <nonsensical>...</nonsensical> tags. Note: Instead of these tags, it would be nicer to return a data structure containing the message string and a boolean indicating whether the message is sensical, but then we cannot use string operations like concatenation, stripMargin etc on the values returned by em"...", and in the current error message composition methods, this is crucial.