Fix checks where error code took precedence over cancellation.

Whenever a function with a cancellation signal argument gets it triggered, it
should err out with `boost::asio::error::operation_aborted` regardless of
other errors, otherwise checks like `ouinet::return_or_throw_on_error` are
going to cause failed assertions.

src/client.cpp

@@ -810,7 +810,7 @@ Session Client::State::fetch_fresh_through_simple_proxy
     // Send request
     http::async_write(con, request, yield[ec].tag("inj-request"));
 
-    if (!ec && cancel_slot) {
+    if (cancel_slot) {
         ec = asio::error::operation_aborted;
     }
 

src/fetch_http_page.h

@@ -38,7 +38,7 @@ fetch_http( Stream& con
 
     // Send the HTTP request to the remote host
     http::async_write(con, req, yield[ec]);
-    if (!ec && cancel_slot) {
+    if (cancel_slot) {
         ec = asio::error::operation_aborted;
     }
     if (ec) {
@@ -55,7 +55,7 @@ fetch_http( Stream& con
 
     // Receive the HTTP response
     _recv_http_response(con, buffer, res, yield[ec]);
-    if (!ec && cancel_slot) {
+    if (cancel_slot) {
         ec = asio::error::operation_aborted;
     }
     if (ec) {

src/util/async_job.h

@@ -72,7 +72,7 @@ public:
             self->_self = nullptr;
             self->_cancel_signal = nullptr;
 
-            if (!ec && cancel) ec = asio::error::operation_aborted;
+            if (cancel) ec = asio::error::operation_aborted;
 
             self->_result = Result{ ec, std::move(retval) };