Set the 'pipefail' option on shell commands

This way we can detect failure of dump / restore commands when used
with the 'pipe' handler (when we're using shell pipes for composition).

shell.go

@@ -78,7 +78,10 @@ func (s *Shell) command(ctx context.Context, arg string) *exec.Cmd {
 	if s.dryRun {
 		args = []string{"/bin/echo", arg}
 	} else {
-		args = []string{"/bin/sh", "-c", arg}
+		// The pipefail option is necessary for us to detect
+		// when the first command in a pipeline fails, but we
+		// need bash for that.
+		args = []string{"/bin/bash", "-o", "pipefail", "-c", arg}
 	}
 
 	if s.niceLevel != 0 {