bubble up the error message from go-getter (#18444)

.changelog/18444.txt

@@ -0,0 +1,3 @@
+```release-note:improvement
+status: go-getter failure reason now shown in `alloc status`
+```

client/allocrunner/taskrunner/getter/util.go

@@ -143,10 +143,11 @@ func (s *Sandbox) runCmd(env *parameters) error {
 
 	// start & wait for the subprocess to terminate
 	if err := cmd.Run(); err != nil {
-		subproc.Log(output, s.logger.Error)
+		msg := subproc.Log(output, s.logger.Error)
+
 		return &Error{
 			URL:         env.Source,
-			Err:         fmt.Errorf("getter subprocess failed: %v", err),
+			Err:         fmt.Errorf("getter subprocess failed: %v: %v", err, msg),
 			Recoverable: true,
 		}
 	}

helper/subproc/subproc.go

@@ -10,6 +10,7 @@ import (
 	"io"
 	"os"
 	"time"
+	"strings"
 )
 
 const (
@@ -45,12 +46,15 @@ func Print(format string, args ...any) {
 //
 // r should be a buffer containing output (typically combined stdin + stdout)
 // f should be an HCLogger Print method (e.g. log.Debug)
-func Log(r io.Reader, f func(msg string, args ...any)) {
+func Log(r io.Reader, f func(msg string, args ...any)) string {
 	scanner := bufio.NewScanner(r)
+	lines := ""
 	for scanner.Scan() {
 		line := scanner.Text()
+		lines += line + "\n"
 		f("sub-process", "OUTPUT", line)
 	}
+	return strings.TrimSpace(lines)
 }
 
 // Context creates a context setup with the given timeout.