return an error instead of panicking when failing to get edge

solver/internal/pipe/pipe.go

@@ -197,3 +197,34 @@ func (pr *receiver) Cancel() {
 func (pr *receiver) Status() Status {
 	return pr.status
 }
+
+type erroredPipe struct {
+	req interface{}
+	err error
+}
+
+func NewErroredPipe(req interface{}, err error) Receiver {
+	return &erroredPipe{
+		req: req,
+		err: err,
+	}
+}
+
+func (p *erroredPipe) Request() interface{} {
+	return p.req
+}
+
+func (p *erroredPipe) Receive() bool {
+	return true
+}
+
+func (p *erroredPipe) Status() Status {
+	return Status{
+		Canceled:  false,
+		Completed: true,
+		Err:       p.err,
+		Value:     nil,
+	}
+}
+
+func (p *erroredPipe) Cancel() {}

solver/scheduler.go

@@ -2,6 +2,7 @@ package solver
 
 import (
 	"context"
+	"fmt"
 	"os"
 	"sync"
 
@@ -351,7 +352,7 @@ type pipeFactory struct {
 func (pf *pipeFactory) NewInputRequest(ee Edge, req *edgeRequest) pipe.Receiver {
 	target := pf.s.ef.getEdge(ee)
 	if target == nil {
-		panic("failed to get edge") // TODO: return errored pipe
+		return pipe.NewErroredPipe(req, fmt.Errorf("failed to get edge"))
 	}
 	p := pf.s.newPipe(target, pf.e, pipe.Request{Payload: req})
 	if debugScheduler {