`Fastfile`: replaced `fail` with `UI.user_error` (#1535)
`Fastfile`: replaced `fail` with `UI.user_error` (#1535)

### Before:
```
[08:26:44]: Error in your Fastfile at line 72
[08:26:44]:     70:	  lane :replace_version_number do |options|
[08:26:44]:     71:	    new_version_number = options[:version]
[08:26:44]:  => 72:	    fail ArgumentError, "missing version" unless new_version_number
[08:26:44]:     73:	    previous_version_number = current_version_number
[08:26:44]:     74:
[08:26:44]: missing version
```

### After:
```
[08:43:52]: fastlane finished with errors

[!] missing version
```
RevenueCat/purchases-ios,RevenueCat/purchases-ios,RevenueCat/purchases-ios,RevenueCat/purchases-ios,RevenueCat/purchases-ios