issue error directly when doc is malformated

This should popup a message saying "No reader engine for this file"
instead of a crash when document file is malformated.

This should fix #868.

frontend/document/pdfdocument.lua

@@ -56,8 +56,7 @@ function PdfDocument:init()
     local ok
     ok, self._document = pcall(pdf.openDocument, self.file, self.mupdf_cache_size)
     if not ok then
-        self.error_message = self._document -- will contain error message
-        return
+        error(self._document)  -- will contain error message
     end
     self.is_open = true
     self.info.has_pages = true
@@ -67,6 +66,9 @@ function PdfDocument:init()
     else
         self:_readMetadata()
     end
+    if not (self.info.number_of_pages > 0) then
+        error("No page found in PDF file")
+    end
 end
 
 function PdfDocument:unlock(password)