Q:

Errors in plugins make a 500 server error with no error message on screen or in log

I have an issue with errors generated in WordPress plugins that is making development difficult for me.
If I trigger an error in a normal PHP file like so:
trigger_error("an error occurred", E_USER_ERROR);

It works fine and I will get the error displayed on the page.  However, if I try the same in the code of a plugin the server generates a 500 error in browser and does not display the error.
In addition, it also does not log any error in the PHP-FPM or Nginx log file when occuring from plugin code.  This is making development extremely difficult as when I get an error I have no information to go on.
As this is working fine in standalone PHP I am assuming it must be WordPress related.
I have debugging enabled in wordpress.  My stack is as follows:

Ubuntu 16.04
PHP 7.0.4
Nginx 1.10.0
MariaDB 10.0.24

The file ownership is user:www-data. Directories are 775 and files are 664
Errors are enabled in the PHP conf files and I can see errors fine with standalone PHP pages, it is just happening on the server in context of errors happening in the WordPress plugins PHP pages.
This is very frustrating as I cannot see why this is happening.  Does anyone have any insight that could help me figure this out?
EDIT
To be more clear here is what I have in wp-config to enable debugging:
define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );

Other debugging values are defaulted to true so this should be all I need.
Curiously I have just noticed that error_log() does not do anything inside a WordPress file while trigger_error() will cause the 500 error.
This feels like some sort of permission based error but I am not sure what.  I really need some help with this as development in this circumstance is practically impossible without any error text.

A:

The problem was caused by a plugin "Wordpress CSV Importer".  I do not know why but removing it made debugging work again.

