Use ErrorHandler to handle proxy error (#1310)

The router prints error no matter what error type it is.
It's useful for troubleshooting, however, it also prints
the context canceled error, which means that users abort
request before reply and its really normal nowadays. Also,
the router returns 502 if error is not nil and may confused
client if it's a timeout error.

To solve these problems, this PR adds an error handler to
reverse proxy to examine the return error and change the
status code when needed.

test/tests/test_function_timeout.sh

@@ -0,0 +1,63 @@
+#!/bin/bash
+
+set -euo pipefail
+source $(dirname $0)/../utils.sh
+
+TEST_ID=$(generate_test_id)
+echo "TEST_ID = $TEST_ID"
+
+tmp_dir="/tmp/test-$TEST_ID"
+mkdir -p $tmp_dir
+
+ROOT=$(dirname $0)/../..
+
+env=nodejs-$TEST_ID
+fn=nodejs-hello-$TEST_ID
+
+export FISSION_ROUTER=localhost:8888
+
+cleanup() {
+    log "Cleaning up..."
+    clean_resource_by_id $TEST_ID
+    rm -rf $tmp_dir
+}
+
+if [ -z "${TEST_NOCLEANUP:-}" ]; then
+    trap cleanup EXIT
+else
+    log "TEST_NOCLEANUP is set; not cleaning up test artifacts afterwards."
+fi
+
+# Create a function in nodejs, test it with an HTTP trigger.
+# Update it and check it's output, the output should be
+# different from the previous one.
+
+log "Creating nodejs env"
+fission env create --name $env --image $NODE_RUNTIME_IMAGE
+
+log "Creating function"
+echo 'function sleep(e){return new Promise(t=>{setTimeout(t,e)})}module.exports=async function(e){return await sleep(5000),{status:200,body:"hello, world!\n"}};' > $tmp_dir/foo.js
+fission fn create --name $fn --env $env --code $tmp_dir/foo.js --fntimeout 10
+
+log "Creating route"
+fission route create --function $fn --url /$fn --method GET
+
+log "Waiting for router to catch up"
+sleep 10
+
+log "Checking for valid response"
+timeout 60 bash -c "test_fn $fn 'hello, world!'"
+
+log "Updating function timeout setting"
+fission fn update --name $fn --fntimeout 2
+
+log "Waiting for router to update cache"
+sleep 10
+
+log "Doing an HTTP GET on the function's route"
+response=$(curl -s -o /dev/null -w "%{http_code}" http://$FISSION_ROUTER/$fn)
+
+log "Checking for status code"
+echo $response | grep -i 504
+
+log "All done."