FailExceptionLike

The base class for all MUnit FailExceptions.

Implementation note: this class exists so that we could fix the issue https://youtrack.jetbrains.com/issue/SCL-18255 In order to support the JUnit comparison GUI in IntelliJ we need to extend org.junit.ComparisonFailure, which is a class and not an interface. We can't make munit.FailException extend org.junit.ComparisonFailure since not all "fail exceptions" are "comparison failures". Instead, we introduced munit.ComparisionFailException, which extends org.junit.ComparisonFailure and this base trait. Internally, MUnit should match against FailExceptionLike[_] instead of munit.FailException directly.