tools/flow: check and return errors from task values

Currently if an identifier doesn't exist within a _tool.cue file,
it's ignored and no error is thrown.

This updates the logic in the flow controller to check and return
errors from task values during initialization.

Fixes #1581.

Change-Id: Iaf1a6262b7f44b07525b9e56079b804047dada9d
Signed-off-by: Nick Figgins <figginsn@gmail.com>

cmd/cue/cmd/testdata/script/cmd_undefined.txtar

@@ -0,0 +1,34 @@
+! exec cue cmd tool
+stderr 'command.tool.bad: undefined field: DoesntExist'
+! exec cue cmd builtin
+stderr 'command.builtin.bad: undefined field: DoesntExist'
+! exec cue cmd package
+stderr 'command.package.bad: undefined field: DoesntExist'
+-- cue.mod/module.cue --
+module: "mod.test/test"
+-- sub/sub.cue --
+package sub
+-- task_tool.cue --
+package home
+
+import (
+    "tool/os"
+    "strconv"
+    "mod.test/test/sub"
+)
+
+command: tool: {
+    bad: os.DoesntExist & {
+        input: "a"
+    }
+}
+
+command: builtin: {
+    bad: strconv.DoesntExist & {
+        a: "b"
+    }
+}
+
+command: package: {
+    bad: sub.DoesntExist
+}

tools/flow/tasks.go

@@ -91,6 +91,10 @@ func (c *Controller) findRootTasks(v cue.Value) {
 
 // getTask finds and marks tasks that are descendents of v.
 func (c *Controller) getTask(scope *Task, v cue.Value) *Task {
+	if err := v.Err(); err != nil {
+		c.addErr(err, "invalid task")
+		return nil
+	}
 	// Look up cached node.
 	_, w := value.ToInternal(v)
 	if t, ok := c.nodes[w]; ok {

tools/flow/testdata/issue2517.txtar

@@ -6,6 +6,7 @@ package x
 root: {
 	prepare: {
 		$id: "prepare"
+		stdout: string
 	}
 	env2: {
 		input: prepare.stdout
@@ -41,7 +42,7 @@ Allocs: 5
 Retain: 0
 
 Unifications: 11
-Conjuncts:    17
+Conjuncts:    20
 Disjuncts:    11
 -- out/run/t2 --
 graph TD
@@ -65,7 +66,7 @@ Allocs: 0
 Retain: 0
 
 Unifications: 12
-Conjuncts:    21
+Conjuncts:    24
 Disjuncts:    12
 -- out/run/stats/totals --
 Leaks:  0
@@ -75,5 +76,5 @@ Allocs: 5
 Retain: 0
 
 Unifications: 23
-Conjuncts:    38
+Conjuncts:    44
 Disjuncts:    23