Merge #670: Backport PR #669

7bbb412 gui: ensure we can retry starting in case of error (jp1ac4)

Pull request description:

ACKs for top commit:
  darosior:
    self-ACK 7bbb412 -- clean backport of #669 ACK'd by Edouard

Tree-SHA512: 151dcb8e5d386312147cde9e562c5aab5add2cfc6a78661f91fa35eab0fb570a03cf03c82e8642f880f6d6ab45810bfca0087789a63601e0cbeb666597f3e020

gui/src/installer/step/bitcoind.rs

@@ -652,9 +652,9 @@ impl Step for InternalBitcoindStep {
                 message::InternalBitcoindMsg::Previous => {
                     if let Some(bitcoind) = &self.internal_bitcoind {
                         bitcoind.stop();
-                        self.started = None;
                     }
                     self.internal_bitcoind = None;
+                    self.started = None; // clear both Ok and Err
                     return Command::perform(async {}, |_| Message::Previous);
                 }
                 message::InternalBitcoindMsg::Reload => {