[BUG] session: errors were not reported in termination flags in TCP mode

[BUG] session: errors were not reported in termination flags in TCP mode

In order to get termination flags properly updated, the session was
relying a bit too much on http_return_srv_error() which is http-centric.

A generic srv_error function was implemented in the session in order to
catch all connection abort situations. It was then noticed that a request
abort during a connection attempt was not reported, which is now fixed.

Read and write errors/timeouts were not logged either. It was necessary
to add those tests at 4 new locations.

Now it looks like everything is correctly logged. Most likely some error
checking code could now be removed from some analysers.