This patch introduces a check_response method to proxy server
Application class. The method provides common checking and reporting
of backend response errors (i.e. responses with status code >=
500). This checking was previously duplicated in proxy controller
classes.
The formatting and content of error logging messages is now more
unform, which does result in changes to some logged error messages.
Change-Id: I4b935a47f39642c11364f2a8d50a5f2ebb3b84f1
623024848e