JSON exceptions

[Lestropie]

If a JSON is malformed, which can occur when manually adding the IntendedFor field to fmap/ files, the resulting Python exception is unhandled, and so a catastrophic crash occurs. It would be preferable to catch such and give a more informative error message.

[treanus]

It also seems that the IntendedFor field can only have 1 entry.
The case "IntendedFor": ["dwi/sub-01_acq-b1000_dwi.nii.gz", "dwi/sub-01_acq-b3000_dwi.nii.gz"] e.g. throws an error.
An fmap could be used for sdc for more than 1 dwi dataset, no?

[Lestropie]

Code here is intended to deal with the prospect of IntendedFor being either a string or a list of strings. If that's not working, that's worthy of its own issue listing.