return pipe error from Wait (#211)

Author: mahadzaryab1

Diff for: README.md

@@ -333,7 +333,7 @@ Sinks are methods that return some data from a pipe, ending the pipeline and ext
 | [`Slice`](https://pkg.go.dev/github.com/bitfield/script#Pipe.Slice) | | data as `[]string`, error  |
 | [`Stdout`](https://pkg.go.dev/github.com/bitfield/script#Pipe.Stdout) | standard output | bytes written, error  |
 | [`String`](https://pkg.go.dev/github.com/bitfield/script#Pipe.String) | | data as `string`, error  |
-| [`Wait`](https://pkg.go.dev/github.com/bitfield/script#Pipe.Wait) | | none  |
+| [`Wait`](https://pkg.go.dev/github.com/bitfield/script#Pipe.Wait) | | error  |
 | [`WriteFile`](https://pkg.go.dev/github.com/bitfield/script#Pipe.WriteFile) | specified file, truncating if it exists | bytes written, error  |
 
 # What's new

Diff for: script.go

@@ -374,6 +374,9 @@ func (p *Pipe) EncodeBase64() *Pipe {
 }
 
 // Error returns any error present on the pipe, or nil otherwise.
+// Error is not a sink and does not wait until the pipe reaches
+// completion. To wait for completion before returning the error,
+// see [Pipe.Wait].
 func (p *Pipe) Error() error {
 	if p.mu == nil { // uninitialised pipe
 		return nil
@@ -874,14 +877,15 @@ func (p *Pipe) Tee(writers ...io.Writer) *Pipe {
 	return p.WithReader(io.TeeReader(p.Reader, teeWriter))
 }
 
-// Wait reads the pipe to completion and discards the result. This is mostly
-// useful for waiting until concurrent filters have completed (see
-// [Pipe.Filter]).
-func (p *Pipe) Wait() {
+// Wait reads the pipe to completion and returns any error present on
+// the pipe, or nil otherwise. This is mostly useful for waiting until
+// concurrent filters have completed (see [Pipe.Filter]).
+func (p *Pipe) Wait() error {
 	_, err := io.Copy(io.Discard, p)
 	if err != nil {
 		p.SetError(err)
 	}
+	return p.Error()
 }
 
 // WithError sets the error err on the pipe.

Diff for: script_test.go

@@ -1850,6 +1850,22 @@ func TestReadReturnsErrorGivenReadErrorOnPipe(t *testing.T) {
 	}
 }
 
+func TestWait_ReturnsErrorPresentOnPipe(t *testing.T) {
+	t.Parallel()
+	p := script.Echo("a\nb\nc\n").ExecForEach("{{invalid template syntax}}")
+	if p.Wait() == nil {
+		t.Error("want error, got nil")
+	}
+}
+
+func TestWait_DoesNotReturnErrorForValidExecution(t *testing.T) {
+	t.Parallel()
+	p := script.Echo("a\nb\nc\n").ExecForEach("echo \"{{.}}\"")
+	if err := p.Wait(); err != nil {
+		t.Fatal(err)
+	}
+}
+
 var base64Cases = []struct {
 	name    string
 	decoded string