Introduce redis connection fault tolerance

With fault tolerance on, which it always is, as it isn't configurable
yet, the value for any operation that raises a connection error is
`nil`. This is intended to make a site operational in the face of a
Redis outage by letting it serve computed content (via `fetch`).

lib/readthis.rb

@@ -16,4 +16,8 @@ module Readthis
   def serializers
     @serializers ||= Readthis::Serializers.new
   end
+
+  def fault_tolerant?
+    true
+  end
 end

lib/readthis/cache.rb

@@ -345,6 +345,8 @@ def invoke(operation, key, &block)
       instrument(operation, key) do
         pool.with(&block)
       end
+    rescue Redis::BaseConnectionError => error
+      raise error unless Readthis.fault_tolerant?
     end
 
     def extract_options!(array)

spec/readthis/cache_spec.rb

@@ -166,6 +166,14 @@ def self.load(value)
       cache.write('great-key', 'great')
       expect(cache.fetch('great-key', nil)).to eq('great')
     end
+
+    it 'serves computed content when the cache is down' do
+      allow(cache.pool).to receive(:with).and_raise(Redis::CannotConnectError)
+
+      computed = cache.fetch('error-key') { 'computed' }
+
+      expect(computed).to eq('computed')
+    end
   end
 
   describe '#read_multi' do