Return a well-defined type when an invalid graph is detected

error.go

@@ -0,0 +1,13 @@
+package workflow
+
+const (
+	_invalidGraphMsg = "dependency graph is unsolvable; check for cycles or missing dependencies"
+)
+
+// An InvalidGraphError is use to mark a dependency graph as being invalid.
+// It is returned when a graph has unmet dependencies or cycles.
+type InvalidGraphError struct{}
+
+func (ige InvalidGraphError) Error() string {
+	return _invalidGraphMsg
+}

workflow.go

@@ -2,7 +2,6 @@ package workflow
 
 import (
 	"context"
-	"errors"
 )
 
 var empty struct{}
@@ -120,7 +119,7 @@ func (g Graph) isWellFormed() error {
 	// once somehow, we either are missing a dependency or we have a cycle.
 	// Either way, we can't execute the graph.
 	if visitedJobs != len(g.tasks) {
-		return errors.New("dependency graph is unsolvable; check for cycles or missing dependencies")
+		return InvalidGraphError{}
 	}
 
 	return nil