Tabular Meta Data File - Cancelation

[jkmarx]

Specific code commit: 7969dc1
Version of the web browser and OS: 49.0.1, 10.11.6 El Capitan
Environment where the error occurred (Vagrant VM and site conf mode or AWS instance): https://test.stemcellcommons.org/#/

Steps to reproduce

Please list all the actions and the input data used:

1. Upload a tabular meta data file
2. Add some large data files
3. During the MD5 calculations, cancel upload

Observed behavior

### Expected behavior

Once cancel upload event occurs, the md5 bloop should go away.

[hackdna]

MD5 calculations for data file uploads should run sequentially