Raise error if -config file has wrong suffix

[valosekj]

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Description

When working with the sct_run_batch script, I incorrectly defined the suffix of the config file to .sh. The error raised by the sct_run_batch script was not very specific. Thus, I added an error raise if the config file is not YML or JSON.

master branch:

$ sct_run_batch -config config.sh

--
Spinal Cord Toolbox (git-master-d6aeaef52466129b5f24a56fa250403839bd0166)

sct_run_batch -config config.sh
--

configuring
Traceback (most recent call last):
  File "/Users/valosek/code/sct_latest/spinalcordtoolbox/scripts/sct_run_batch.py", line 597, in <module>
    main(sys.argv[1:])
  File "/Users/valosek/code/sct_latest/spinalcordtoolbox/scripts/sct_run_batch.py", line 359, in main
    config_keys = set(config.keys())
UnboundLocalError: local variable 'config' referenced before assignment

jv/raise_error_if_sct_run_batch_config_has_wrong_suffix branch:

$ sct_run_batch -config config.sh

--
Spinal Cord Toolbox (git-master-d6aeaef52466129b5f24a56fa250403839bd0166*)

sct_run_batch -config config.sh
--

configuring
Traceback (most recent call last):
  File "/Users/valosek/code/sct_latest/spinalcordtoolbox/scripts/sct_run_batch.py", line 599, in <module>
    main(sys.argv[1:])
  File "/Users/valosek/code/sct_latest/spinalcordtoolbox/scripts/sct_run_batch.py", line 353, in main
    raise ValueError('Unrecognized configuration file type: {}'.format(ext))
ValueError: Unrecognized configuration file type: .sh

[mguaypaq]

Thanks for converting this to an if...elif...else pattern! That's much better.