[damned-lies] Prevent crash when build error string is not valid unicode

[Claude Paroz <claudep src gnome org>]

commit 674e056c7d280687a8326300da68136611741ec6
Author: Claude Paroz <claude 2xlibre net>
Date:   Wed May 29 21:16:49 2019 +0200

    Prevent crash when build error string is not valid unicode

 vertimus/views.py | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)
---
diff --git a/vertimus/views.py b/vertimus/views.py
index b821ac8b..9f9fc3b1 100644
--- a/vertimus/views.py
+++ b/vertimus/views.py
@@ -339,7 +339,8 @@ class BuildTranslatedDocsView(PoFileActionBase):
             ], stderr=subprocess.PIPE)
             if result.returncode != 0:
                 return build_error % {
-                    'program': 'msgfmt', 'err': result.stderr.decode()
+                    'program': 'msgfmt',
+                    'err': result.stderr.decode(errors='replace'),
                 }
 
             sources = doc_format.source_files()