Improve the error message on the assignments page in the case that the exchange does not exist

Author: enisnazif

nbgrader/server_extensions/assignment_list/handlers.py

@@ -16,7 +16,7 @@
 
 from ...apps import NbGrader
 from ...coursedir import CourseDirectory
-from ...exchange import ExchangeList, ExchangeFetchAssignment, ExchangeFetchFeedback, ExchangeSubmit
+from ...exchange import ExchangeList, ExchangeFetchAssignment, ExchangeFetchFeedback, ExchangeSubmit, ExchangeError
 from ...auth import Authenticator
 from ... import __version__ as nbgrader_version
 
@@ -76,11 +76,16 @@ def list_released_assignments(self, course_id=None):
                     config=config)
                 assignments = lister.start()
 
-            except:
+            except ExchangeError:
                 self.log.error(traceback.format_exc())
                 retvalue = {
                     "success": False,
-                    "value": traceback.format_exc()
+                    "value": """The exchange directory does not exist and could
+                                not be created. The "release" and "collect" functionality will not be available.
+                                Please see the documentation on
+                                http://nbgrader.readthedocs.io/en/stable/user_guide/managing_assignment_files.html#setting-up-the-exchange
+                                for instructions.
+                            """
                 }
 
             else:
@@ -111,11 +116,16 @@ def list_submitted_assignments(self, course_id=None):
                     config=config)
                 assignments = lister.start()
 
-            except:
+            except ExchangeError:
                 self.log.error(traceback.format_exc())
                 retvalue = {
                     "success": False,
-                    "value": traceback.format_exc()
+                    "value": """The exchange directory does not exist and could
+                                not be created. The "release" and "collect" functionality will not be available.
+                                Please see the documentation on
+                                http://nbgrader.readthedocs.io/en/stable/user_guide/managing_assignment_files.html#setting-up-the-exchange
+                                for instructions.
+                            """
                 }
 
             else: