Instead of exiting, disable optional eCAP services that fail initialization.

Instead of exiting, disable optional eCAP services that fail initialization.

Report all service initialization failures but mark optional services
down instead of killing Squid by propagating the failure to the Squid
core.  An initialization failure of an optional (bypass=1) service
should not lead to Squid quitting because, according to squid.conf, such
a service may be replaced by other services (using adaptation sets) or
simply ignored.

Initialization failures of essential services still lead to fatal
errors, but they are now reported better.

Ecap services should indicate failures by throwing an exception. Squid
reports exception details using std::exception::what() method if
possible.