When incorrect data is uploaded to be redacted, no error is provided to the user

[AdamLeonSmith]

Steps to reproduce:

• Open the redaction tool
• Upload a JSON file that is not in the right format

What happened:
The sea is shown in the map, but no error is visible to the user. Errors are thrown to the JS console e.g.

Uncaught SyntaxError: Unexpected token , in JSON at position 281
at JSON.parse ()
at FileReader. (index.html:1104)

or

Uncaught TypeError: Cannot read property 'time' of undefined
at FileReader. (index.html:1427)

What I expected to happen
I expected to see an error that is visible to the user, and advises them that the file format is incorrect.