turn missing dependencies in nbconvert to 500 errors

pygments is the only such example at this time

IPython/html/nbconvert/handlers.py

@@ -61,7 +61,10 @@ def get_exporter(format, **kwargs):
         # should this be 400?
         raise web.HTTPError(404, u"No exporter for format: %s" % format)
     
-    return Exporter(**kwargs)
+    try:
+        return Exporter(**kwargs)
+    except Exception as e:
+        raise web.HTTPError(500, "Could not construct Exporter: %s" % e)
 
 class NbconvertFileHandler(IPythonHandler):
 

IPython/html/services/nbconvert/handlers.py

@@ -3,10 +3,6 @@ import json
 from tornado import web
 
 from ...base.handlers import IPythonHandler, json_errors
-try:
-    from IPython.nbconvert.exporters.export import exporter_map
-except ImportError:
-    exporter_map = {}
 
 class NbconvertRootHandler(IPythonHandler):
     SUPPORTED_METHODS = ('GET',)
@@ -14,6 +10,10 @@ class NbconvertRootHandler(IPythonHandler):
     @web.authenticated
     @json_errors
     def get(self):
+        try:
+            from IPython.nbconvert.exporters.export import exporter_map
+        except ImportError as e:
+            raise web.HTTPError(500, "Could not import nbconvert: %s" % e)
         res = {}
         for format, exporter in exporter_map.items():
             res[format] = info = {}