diff --git a/src/calibre/gui2/viewer/convert_book.py b/src/calibre/gui2/viewer/convert_book.py
index c212b9669f..cb443c0a75 100644
--- a/src/calibre/gui2/viewer/convert_book.py
+++ b/src/calibre/gui2/viewer/convert_book.py
@@ -120,18 +120,28 @@ def prepare_convert(temp_path, key, st):
     }
 
 
+class ConversionFailure(ValueError):
+
+    def __init__(self, book_path, worker_output):
+        self.book_path = book_path
+        self.worker_output = worker_output
+        ValueError.__init__(
+                self, 'Failed to convert book: {} with error:\n{}'.format(book_path, worker_output))
+
+
 def do_convert(path, temp_path, key, instance):
     tdir = os.path.join(temp_path, instance['path'])
-    with TemporaryFile('log.txt') as logpath, open(logpath, 'w+b') as logf:
-        p = start_pipe_worker('from calibre.srv.render_book import viewer_main; viewer_main()', stdout=logf, stderr=logf)
-        p.stdin.write(msgpack_dumps((
-            path, tdir, {'size': instance['file_size'], 'mtime': instance['file_mtime'], 'hash': key},
-            )))
-        p.stdin.close()
-    if p.wait() != 0:
-        with lopen(logpath, 'rb') as logf:
-            raise Exception('Failed to convert book: {} with errors:\n{}'.format(
-                path, logf.read().decode('utf-8', 'replace')))
+    with TemporaryFile('log.txt') as logpath:
+        with open(logpath, 'w+b') as logf:
+            p = start_pipe_worker('from calibre.srv.render_book import viewer_main; viewer_main()', stdout=logf, stderr=logf)
+            p.stdin.write(msgpack_dumps((
+                path, tdir, {'size': instance['file_size'], 'mtime': instance['file_mtime'], 'hash': key},
+                )))
+            p.stdin.close()
+        if p.wait() != 0:
+            with lopen(logpath, 'rb') as logf:
+                worker_output = logf.read().decode('utf-8', 'replace')
+            raise ConversionFailure(path, worker_output)
     size = 0
     for f in walk(tdir):
         size += os.path.getsize(f)
diff --git a/src/calibre/gui2/viewer/ui.py b/src/calibre/gui2/viewer/ui.py
index 2a181b09d8..3726c02da0 100644
--- a/src/calibre/gui2/viewer/ui.py
+++ b/src/calibre/gui2/viewer/ui.py
@@ -6,6 +6,7 @@ from __future__ import absolute_import, division, print_function, unicode_litera
 
 import json
 import os
+import re
 import sys
 from collections import defaultdict, namedtuple
 from hashlib import sha256
@@ -333,14 +334,15 @@ class EbookViewer(MainWindow):
         open_at, self.pending_open_at = self.pending_open_at, None
         if not ok:
             self.setWindowTitle(self.base_window_title)
-            tb = data['tb']
+            tb = data['tb'].strip()
+            tb = re.split(r'^calibre\.gui2\.viewer\.convert_book\.ConversionFailure:\s*', tb, maxsplit=1, flags=re.M)[-1]
             last_line = tuple(tb.strip().splitlines())[-1]
             if last_line.startswith('calibre.ebooks.DRMError'):
                 DRMErrorMessage(self).exec_()
             else:
                 error_dialog(self, _('Loading book failed'), _(
                     'Failed to open the book at {0}. Click "Show details" for more info.').format(data['pathtoebook']),
-                    det_msg=data['tb'], show=True)
+                    det_msg=tb, show=True)
             self.loading_overlay.hide()
             self.web_view.show_home_page()
             return