Fix rescue clause in backpressure prose test

Mongo::Error::PoolTimeout does not exist; replace with StandardError
which covers all connection-related errors (SocketError, pool checkout
timeouts, NoServerAvailable, etc.).

Author: comandeo-mongo

spec/integration/sdam_prose_spec.rb

@@ -166,9 +166,7 @@
             client.use('test')['test'].find(
               '$where' => 'function() { sleep(2000); return true; }'
             ).first
-          rescue Mongo::Error::PoolTimeout,
-                 Mongo::Error::SocketError,
-                 Mongo::Error::NoServerAvailable
+          rescue StandardError
             # Ignore connection errors (including checkout timeouts).
           end
         end