Fix error handling in BlockDownloaderImpl

`lastError` inside the `BlockDownloaderImpl` was set even when
downloading task was canceled. And because it was canceled the
`lastError` has never been read by CBP before the next sync. And when
next sync was started it was immediately failed because of cancelation
error stored in `lastError`.

Sources/ZcashLightClientKit/Block/Download/BlockDownloader.swift

@@ -139,10 +139,10 @@ actor BlockDownloaderImpl {
                 isDownloading = false
             }
         } catch {
-            lastError = error
             if Task.isCancelled {
                 logger.debug("Blocks downloading canceled.")
             } else {
+                lastError = error
                 logger.error("Blocks downloading failed: \(error)")
             }
             isDownloading = false