Moved string contain matcher
Changed matcher continuation to include actual value

This allows different matchers to modify what is considered the actual
value. E.g. if the matcher input is a function, and the matcher expects an
exception with a specific message, then the matcher failure message should
display the actual exception message, and not the matcher input, which is
the function expected to throw an exception.
Display actual exception message when match fails

When a match for an expected exception message fails, it is helpful to know
what the actual exception message was