Do not calculate layout, used saved layout instead (#343)

* Do not calculate layout, used saved layout instead

See for rationale: https://docs.google.com/document/d/1HJFvbdDHxxgkCW0DZzdTZ9CUMgc2ef2rQFukATBQWvE/edit

* Error handling for when layout has not been precomputed

* Server error (500) not client error (400) for unprepared data

server/app/util/utils.py

@@ -30,6 +30,12 @@ class InteractiveError(Exception):
         self.message = message
 
 
+class PrepareError(Exception):
+
+    def __init__(self, message):
+        self.message = message
+
+
 def get_mime_type(default="application/json", acceptable_types=["application/json", "text/csv"], query_param=None,
                   header=None):
     mime_type = default