[fuchsia] Only write boot image signature on download success

Previously, the file was always written, even in the case of an
exception.

Change-Id: I0cee215b529f6864c8c14fc90934de2fc7f44974
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2466738Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Commit-Queue: David Dorwin <ddorwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817297}

build/fuchsia/update_images.py

@@ -128,13 +128,13 @@ def main():
     try:
       DownloadSdkBootImages(bucket, sdk_hash, args.boot_images,
                             args.image_root_dir)
+      with open(signature_filename, 'w') as f:
+        f.write(new_signature)
+
     except subprocess.CalledProcessError as e:
       logging.error(("command '%s' failed with status %d.%s"), " ".join(e.cmd),
                     e.returncode, " Details: " + e.output if e.output else "")
 
-    with open(signature_filename, 'w') as f:
-      f.write(new_signature)
-
   return 0