Make MismatchError a base Exception

This allows mismatches to avoid being caught by bare rescue's which
should help make them more visible in the face of code which chooses to
use such a general rescue strategy.

Author: iamvery

lib/scientist/experiment.rb

@@ -18,7 +18,7 @@ def self.new(name)
   end
 
   # A mismatch, raised when raise_on_mismatches is enabled.
-  class MismatchError < StandardError
+  class MismatchError < Exception
     attr_reader :name, :result
 
     def initialize(name, result)