Revert "tools/flow: check and return errors from task values"

This reverts commit 9ef35eb
but keeps the `cmd_undefined` test case and updates
the `issue2517` test case so that it illustrates the way that
the reverted commit fails.

That commit introduced a regression that caused existing
tools/flow uses to fail. See https://cuelang.org/issue/2965.

A subsequent CL will apply a different fix for #1581.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I322ac3b269bc79a541c15001a166160ad9191267

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

@@ -1,9 +1,18 @@
-! 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'
+# This test illustrates the problem demonstrated
+# in https://github.com/cue-lang/cue/issues/1581
+# TODO fix tools/flow so it fails in this case.
+
+# TODO this should not succeed
+exec cue cmd tool
+# stderr 'command.tool.bad: undefined field: DoesntExist'
+
+# TODO this should not succeed
+exec cue cmd builtin
+# stderr 'command.builtin.bad: undefined field: DoesntExist'
+
+# TODO this should not succeed
+exec cue cmd package
+# stderr 'command.package.bad: undefined field: DoesntExist'
 -- cue.mod/module.cue --
 module: "mod.test/test"
 -- sub/sub.cue --

tools/flow/tasks.go

@@ -91,10 +91,6 @@ 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

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