catch in createIndexPlugin() obscures errors

Problem/Motivation

createIndexPlugin() does a catch and doesn't pass on the error:

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

That means that if the exception is caused by an error in code rather than by a missing plugin, you don't understand what's going on.

Steps to reproduce

Proposed resolution

Remaining tasks