[ExternalExtractor] Provide more helpful output when extractor fails

Summary:
In case the executable fails to start, no error was given at all.
Show the pluginpath if no manifest.json is found.
Show the extractor path and indexed file when the extractor fails at
runtime.

Test Plan: ctest

Reviewers: Baloo, Frameworks, ngraham, poboiko, astippich, lbeltrame

Reviewed By: lbeltrame

Subscribers: lbeltrame, kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D19108