Change failure message for BeComparedTo#failure_message_when_negated

- Normal error message will be given for all operator except comparison
  operators
- For comparison operators such as gt/lt/gte/lte, with normal message a
  warning saying that the usage is confusing is given.
- "It might be more clearly expressed without the" is dropped completely
  as it doesn't sound correct with except
- Fixes #386
Merge pull request #417 from prathamesh-sonpatki/issue-386

Change failure message for BeComparedTo#failure_message_when_negated