Fixe a regression in 2.14.6 w/ Fivemat formatter.

In https://github.com/tpope/fivemat/blob/v1.2.1/lib/fivemat/rspec.rb#L43-L49,
Fivemat has two ways to check for pending example fixed:
 - example.execution_result[:exception].pending_fixed?
   (but we removed this in #1106 because it broke when dealing with frozen exceptions)
 - RSpec::Core::PendingExampleFixedError === exception
   (but this const was renamed in b5d10ccbe3a590adc67d807acb565be3657e51bf).

The `RSpec::Core::PendingExampleFixedError` line in Fivemat was
never hit in recent releases because `example.execution_result[:exception]`
responded to `pending_fixed?` -- until 2.14.6.

@dchelimsky -- do you remember the motivation for renaming the error class in b5d10ccbe3a590adc67d807acb565be3657e51bf ?  The commit message doesn't give a rationale.  I don't think this should break anything (and fixes the Fivemat problem) but wanted to check with you.