Prevent fatals upon plugin activation

This object cache is distributed on https://wordpress.org/plugins/memcached/ as a plugin, but can't actually be used as one. The [installation instructions](https://wordpress.org/plugins/memcached/#installation) do suggest it requires extra configutaration, but since most plugins only have junk there, no-one looks at it.

Due to the number of end-users who attempt to install it [(See the support threads)](https://wordpress.org/support/plugin/memcached/), only to be met with a fatal error, we should disable the plugin activation aspect of the plugin and present a useful error message to the end-user, rather than simply showing a fatal error.

I'm not sure of the best method to achieve this, but if it's going to be on WordPress.org/plugins it would be great to remove that "limitation".

Start by reviewing the plugin activation path and the linked WordPress.org installation instructions, then trace where activation currently reaches the fatal error. Done means an unconfigured installation is prevented from activating and shows a useful error message instead of a fatal error.

Written by the indexing model from the issue text.