When uploading a too large image, no error is shown and when submitted a 403 forbidden is returned

[Fastjur]

Describe the bug
There is no feedback in the frontend of the page regarding the limit of the size of an image. If a too large image is uploaded there is no visual feedback that it is too large and will be rejected. Once one clicks on submit a 403 is returned and most of the work is lost when navigating back.

To Reproduce
Steps to reproduce the behavior:

1. https://ch.tudelft.nl/events/administrator
2. Click on '+ NEW'
3. Add some testing data, and upload an image that is too large (> 1 MB)
4. Click on 'SAVE EVENT'
5. 403 forbidden is returned

Expected behavior
An error is shown indicating the image is too large and it is not allowed to click on SAVE EVENT

Desktop (please complete the following information):

• OS: macOS High Sierra 10.13.5
• Browser Safari
• Version 11.1.1 (13605.2.8)

Additional context
I would also like the maximum size increased, as 1MB is quite low. I am opening a second issue about this (#271)