Remove all direct use of ghost script, add PDFTOPPM_PATH to allow configurable pdftoppm path, raise ConvertError on coverterror and not UnknownFile exception blindly

docs/topics/settings.rst

@@ -187,6 +187,13 @@ Default: ``/usr/bin/libreoffice``
 Path to the libreoffice binary used to call LibreOffice for office document conversion.
 
 
+**CONVERTER_PDFTOPPM_PATH**
+
+Default: ``/usr/bin/pdftoppm``
+
+Path to the Popple program pdftoppm.
+
+
 Storage
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