Check for /data/import folder

A common problem after updating translate5 to the version with the TermPortal is a message complaining that "something is not writeable".
Reason is that /data/import is not exisiting or not writeable.
Please implement a dir_exists and is_writable check in the TBX import. Should be before the mkdir which creates the TBX import folders inside /data/import/.
If it does not exist or is not writeable should trigger understandable error messages.