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

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.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25632 3bd1e089-480b-0410-8dfb-8563597acbee