Throw exception if file_put_contents failed

[dmitryuk]

If psalm.xml is not writable, commands vendor/bin/psalm-plugin enable|disable %some_plugin will always response

 [OK] Plugin enabled 

with warning:

Warning: file_put_contents(/var/www/psalm.xml): Failed to open stream: Permission denied in /var/www/vendor/vimeo/psalm/src/Psalm/Internal/PluginManager/ConfigFile.php on line 146

For disabled warnings (error_reporting=E_ERROR) a coder will success without writing the result.
The PR throws an exceptions this case

[weirdan]

Thanks!