ID 423591 - [C] Export: No error shown to the end user when QuickTime fails to render

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Problem summary
The Flix Client does not show an error when the QuickTime fails to generate.

Customer reported version
6.3.3

Steps to reproduce 
Warning: We strongly recommend not taking these steps on a production Server.
1) Restrict your Flix Server's /tmp directory to only allow up to 1GB.
2) Via the Flix Client, find a Sequence Revision that has over 2,600 frames, with a mix of animated and non-animated panels.
3) Attempt to export a QuickTime for this Sequence Revision. Wait at least 5 minutes, after a while you should get an error on the Server stating "no space left on device", followed by the line below, marking the end of the QuickTime creation:
time="02 Sep 20 16:52 BST" level=error msg="job failed, stopping chain" func="chain.(*JobChain).processJob:" Job="Create Quicktime job" error="write /tmp/Flix022473267/097617484/5392.png: no space left on device"
Note: There can be other reasons why a QuickTime fails to generate.
 
Expected behaviour
The Client should give an error telling the user the QuickTime failed to be created, something along the lines of "Flix Server failed to generate the QuickTime with error '[insert server error here]'. Refer to the Server logs for more information".

Actual behaviour
The Client provides no error, and hangs with the Export popup open, and the spinning wheel at its bottom right corner. 

Reproduced by support
This bug has been reproduced in:
6.3.4 - macOS 10.14 - CentOS 7 Server
6.3.0 - macOS 10.14 - CentOS 7 Server
Older versions may still reproduce the issue but have not been tested

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