PluginHelper::createIndexPlugin() should pass on message from PluginException

Problem/Motivation

PluginHelper::createIndexPlugin() ignores the caught PluginException's message:

    catch (PluginException) {
      throw new SearchApiException("Unknown $type plugin with ID '$plugin_id'");

However, in the case that a plugin ID is not found, PluginException now includes a helpful message which lists the existing plugin IDs. This is very useful when you've made a typo!

Steps to reproduce

Proposed resolution

Use the message from the caught exception as the message in the new one.

Remaining tasks