--- a/odfcast/convert.py	Mon Oct 20 16:16:53 2014 +0200
+++ b/odfcast/convert.py	Wed Oct 22 14:38:31 2014 +0200
@@ -27,6 +27,36 @@
 DEFAULT_MIMETYPE = "application/octet-stream"
 
 
+class ErrorResponse(object):
+
+    def __init__(self, title, details="", html_error_code=500):
+        self.title = title
+        self.details = details
+        self.html_error_code = html_error_code
+
+    def __call__(self):
+        if self.is_wants_json():
+            return self.json()
+        else:
+            return self.html()
+
+    def json(self):
+        return json.dumps({
+            "error": self.title,
+            "details": self.details
+        }), self.html_error_code
+
+    def html(self):
+        return self.title, self.html_error_code
+
+    def is_wants_json(self):
+        best = request.accept_mimetypes.best_match(['application/json',
+                                                    'text/html'])
+        return best == 'application/json' and \
+            request.accept_mimetypes[best] > \
+            request.accept_mimetypes['text/html']
+
+
 class ConvertView(MethodView):
 
     def __init__(self, pyuno_driver_name="", hostname="localhost", port=2001):
@@ -43,11 +73,15 @@
     def post(self):
         ffile = request.files['file']
         if not ffile.filename:
-            return "Please upload a file for conversion", 401
+            error = ErrorResponse("Please upload a file for conversion",
+                                  html_error_code=401)
+            return error()
 
         fformat = request.form['format']
         if not self.is_format_supported(fformat):
-            return "Format %s not allowed" % fformat, 401
+            error = ErrorResponse("Format %s not allowed" % fformat,
+                                  html_error_code=401)
+            return error()
 
         datadict = self.get_datadict()
 
@@ -58,20 +92,24 @@
         if datadict:
             try:
                 tfile = tempfile.NamedTemporaryFile()
-                t = Template(outfile, tfile)
+                t = Template(outfile, tfile, ignore_undefined_variables=True)
                 t.render(datadict)
                 outfile.close()
                 outfile = tfile
-            except:
+            except Exception, e:
                 log.exception("Template error")
-                return "Template error", 500
+                error = ErrorResponse(
+                    "Template error", details=str(e), html_error_code=500)
+                return error()
 
         if fformat != "odt":
             try:
                 outfile = self.convert(outfile, fformat)
-            except:
+            except Exception, e:
                 log.exception("Conversion error")
-                return "Conversion error", 500
+                error = ErrorResponse(
+                    "Conversion error", details=str(e), html_error_code=500)
+                return error()
 
         return Response(outfile, mimetype=mimetype)