BUG: fix runtime error related to creation of DICOM database directory

Created by: fedorov

In some situations, user settings may point to a temporary directory with multiple nesting levels, which cannot be created by os.mkdir().

To solve the issue, when exception occurs, fall back to creating the DICOM DB directory in the default location.

Also fix a typo for the name of the DICOM DB location in developer mode.

cc: @che85