Cancelling an image upload in building management is not handled

**[Original report](https://bitbucket.org/laborstudents/cas-flask/issue/373) by Scott Heggen (Bitbucket: [sheggen](https://bitbucket.org/sheggen), GitHub: [sheggen](https://github.com/sheggen)).**

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Files: buildingManagement.py

UI: /buildingManagement

When a user is in the process of uploading an image using the Dropzone widget, they have the choice to cancel the upload while it’s in progress. Our controller doesn’t handle this issue, but instead throws a 400 bad request error. The error has no negative impact on the application, as far as I could tell.