Don't blow up when generating a failure message involving stdout/stderr.
Fix `change` matcher to work properly with `IO` objects.
Don't raise errors in exists matcher

Fail gracefully instead
Merge pull request #508 from rspec/fix-io-error

Don't blow up when generating a failure message involving stdout/stderr.
Merge pull request #511 from danielfone/composable-exists-matcher

Don't raise errors in exists matcher
Add a changelog entry for #511.
Improve wording a bit.