[imaging_uploader] Auto-launch failure not reported in server process manager module

[nicolasbrossard]

This PR fixes both the imaging uploader and server process manager module so that the test case #19 in the imaging uploader's test plan succeeds:

Set the config setting 'ImagingUploader auto launch' to 'Yes'. Then, edit your prod file (in <LORIS MRI code dir>/dicom-archive/.loris-mri) and comment out the definition of the @db array. Once these operations are done, upload any valid scan: check in the server processes manager that this fails with an error code of 4. Now try to upload the scan again. When the system asks you if you want to overwrite the existing archive, answer 'Yes'. Check that this reupload fails with error code 4 (and not 2). Related to Redmine#14093 and PR#3555. [Manual Testing]

Fixes #8822