Only reset the server error if there was an error to raise - fix race condition

lib/capybara/session.rb

@@ -123,17 +123,19 @@ module Capybara
     # Raise errors encountered in the server
     #
     def raise_server_error!
-      if Capybara.raise_server_errors and @server and @server.error
+      if @server and @server.error
         # Force an explanation for the error being raised as the exception cause
         begin
-          raise CapybaraError, "Your application server raised an error - It has been raised in your test code because Capybara.raise_server_errors == true"
+          if Capybara.raise_server_errors
+            raise CapybaraError, "Your application server raised an error - It has been raised in your test code because Capybara.raise_server_errors == true"
+          end
         rescue CapybaraError
           #needed to get the cause set correctly in JRuby -- otherwise we could just do raise @server.error
           raise @server.error, @server.error.message, @server.error.backtrace
+        ensure
+          @server.reset_error!
         end
       end
-    ensure
-      @server.reset_error! if @server
     end
 
     ##