qute-keepass: Add missing exit

Closes #6592

doc/changelog.asciidoc

@@ -25,6 +25,7 @@ Fixed
 - Switching tabs via mouse wheel scrolling now works properly on macOS. Set
   `tabs.mousewheel_switching` to false if you prefer the previous behavior.
 - Crash when entering unicode surrogates into the filename prompt.
+- `UnboundLocalError` in `qute-keepass` when the database couldn't be opened.
 
 Changed
 ~~~~~~~

misc/userscripts/qute-keepass

@@ -178,6 +178,7 @@ def find_candidates(args, host):
         kp = pykeepass.PyKeePass(file_path, password=pw, keyfile=kf)
     except Exception as e:
         stderr("There was an error opening the DB: {}".format(str(e)))
+        sys.exit(ExitCodes.DB_OPEN_FAIL)
 
     return kp.find_entries(url="{}{}{}".format(".*", host, ".*"), regex=True)