Improved the error messages that occur when working with a file system that has lost write permission.  Also, if an upload that was started is in the resume state and is dismissed, try to delete the abandoned upload.
I noticed that we weren't actually testing chunked uploads to non-S3 assetstores.  I refactored the test code to do so and to allow testing with smaller files for chunked uploads.
Added tests for editing and deleting an item.
Added a test for aborting an upload.
Removed a debug statement I had left in the test code.
500 errors shouldn't use RestExceptions.
Merge branch 'master' into filesystem-write-permissions

Conflicts:
	tests/mock_s3.py
Merge pull request #401 from girder/filesystem-write-permissions

Filesystem write permissions