Improve error reporting when invalid identifiers are passed by the user

[fedorov]

Current error reporting is completely inadequate and does not help. At the same time, it rather easy to get confused and put series UID in place of study, for example, and get the error below.

[fedorov]

I was also thinking we could have an API endpoint that would automatically try to figure out what the identifier passed is - collection, patient, study, series - notify the user when a match is found and what is matched, and download the corresponding data. This might be more convenient and robust, but potentially will prevent users from trying to appreciate the data model.