Builds a textual description of a specific mismatch.
item is the value that was tested by matches;
the Map that was passed to and supplemented by matches
with additional information about the mismatch, and
is the Description that is being built to describe the mismatch.
A few matchers make use of the
verbose flag to provide detailed
information that is not typically included but can be of help in
diagnosing failures, such as stack traces.
Description describeMismatch( item, Description description, Map matchState, bool verbose) => StringDescription(matchState[this] as String);