Checks for the presence of saved data before warning about missing trigger file

[alexpiet]

Pull Request instructions:

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Describe changes:

Right now the GUI generates a warning if there is a missing camera trigger file. However if no video data was collected, then this file wont exist. Now we check for saved video data before issuing a warning. If saved video data does exist, the warning is logged as an error

What issues or discussions does this update address?

• resolves #442

Describe the expected change in behavior from the perspective of the experimenter

• Removes confusing warning message in 447 rigs

Describe any manual update steps for task computers

• none

Was this update tested in 446/447?

• [x] tested in 447

[alexpiet]

@hanhou @XX-Yin This is ready for review