Git rid of string exceptions

[ylep]

String exceptions (raise "string") have been deprecated since long, and do not work under Python 3.

Strangely, flake8 does not seem to have a way of detecting those, so we will need to use grep or similar tools.

• [x] casa-distro
• [x] brainvisa-cmake
• [x] soma-base
• [x] soma-workflow
• [x] capsul
• [x] axon
• [x] aims-free
• [x] aims-gpl
• [x] anatomist-free
• [x] anatomist-gpl
• [x] morphologist
• [x] morphologist-ui
• [x] morpho-deepsulci
• [x] cortical_surface
• [x] brainrat
• [x] disco
• [x] constellation
• [x] nuclear-imaging-private
• [x] nuclear-imaging-gpl
• [x] highres-cortex
• [x] datamind
• [x] brainvisa-spm
• [x] data_storage_client
• [x] fmri
• [x] longitudinal_pipelines
• [x] snapbase