ErrorFormatter Typedef
The ErrorFormatter type is used for functions that
can be used to build up error reports upon expect failures.
There is one built-in implementation (defaultErrorFormatter)
which is used by the default failure handler. If the failure handler
is replaced it may be desirable to replace the stringDescription
error formatter with another.
Code
typedef String ErrorFormatter(actual, Matcher matcher, String reason,
MatchState matchState, bool verbose);