raise explicit error if file fails to derive

This provides more visibility if further OOM errors cause the process to
fail silently

lib/space_stone/ocrcelot.rb

@@ -22,9 +22,11 @@ def ocr
       FileUtils.mkdir_p(File.dirname(hocr_path))
       cmd = "#{OCR_CMD} '#{mono_path}' #{hocr_path} hocr"
       run(cmd)
-      puts "remove tmp files:"
-      puts %x{rm -v #{mono_path}}
-      "#{hocr_path}.hocr"
+      file_path = "#{hocr_path}.hocr"
+      raise 'generating OCR file failed' unless File.exist?(file_path)
+      puts 'remove tmp files:'
+      puts `rm -v #{mono_path}`
+      file_path
     end
 
     def prep_file
@@ -34,6 +36,7 @@ def prep_file
       opts = '-depth 1 -monochrome -compress Group4 -type bilevel'
       cmd = "convert '#{path}' #{opts} '#{out_path}'"
       run(cmd)
+      raise 'generating TIFF file failed' unless File.exist?(out_path)
       out_path
     end
 

lib/space_stone/thumbnail_service.rb

@@ -26,6 +26,7 @@ def derive
       cmd = "convert #{path} -thumbnail '200x150>' -flatten #{thumbnail_path}"
       puts cmd
       `#{cmd}`
+      raise 'generating thumbnail failed' unless File.exist?(thumbnail_path)
       thumbnail_path
     end
   end