Load modules from the mongo modulestore as errordescriptors if creating a regular descriptor fails, but leave the location the same

Make imports into the CMS ignore broken modules, while still loading bad xml as error modules in the LMS

Make ErrorDescriptor continue to use the interface for __init__ that XModuleDescriptor uses, while providing a helper function for constructing from a different set of arguments

Move common code for updating modules in mongo to a single helper function that also handles creating the item if it doesn't already exist

Define 'system metadata' which should not be editable by the end user. When posting back metadata edits, we need to fetch a copy of the existing metadata and apply the changes.

* for more context, see https://basecamp.com/1892446/projects/631155-devops/messages/4719963-strange-caching

Put SequenceDescriptor back as a subclass of MakeModuleDescriptor. THe previous change to have it derive from EditorDescriptor was not safe as EditorDescriptor presumes that there is a edit box in the JS save flows. So we need to edit get_context() on MakoModuleDescriptor to also pass along the metadata to the Mako rendering templates