roachtest: add Tasker error handling comment

Added a comment to inform test implementors, using the task API, that returning
an error from a Tasker goroutine will fail a test.

Informs: cockroachdb#118214

Epic: None
Release note: None

pkg/cmd/roachtest/roachtestutil/task/tasker.go

@@ -16,9 +16,12 @@ type Func func(context.Context, *logger.Logger) error
 // Tasker is an interface for executing tasks (goroutines). It is intended for
 // use in tests, enabling the test framework to manage panics and errors.
 type Tasker interface {
-	// Go runs the given function in a goroutine.
+	// Go runs the given function in a goroutine. If an error is returned, it will
+	// fail the test. Panics are recovered and treated as errors.
 	Go(fn Func, opts ...Option)
 	// GoWithCancel runs the given function in a goroutine and returns a
-	// CancelFunc that can be used to cancel the function.
+	// CancelFunc that can be used to cancel the function. If an error is
+	// returned, it will fail the test. Panics are recovered and treated as
+	// errors.
 	GoWithCancel(fn Func, opts ...Option) context.CancelFunc
 }