uhd_images_downloader fixes/improvements

* Permissions checking leads to clearer errors when user doesn't have write permissions
* Relative paths work
uhd_images_downloader: more improvements/fixes

* Better error handling
* Improved default download location logic
uhd_images_downloader: redundancy fixes, better error handling
Merge branch 'maint' into ncorgan/images_downloader_fixes
uhd_images_downloader: more descriptive error when MD5 check fails
Merge branch 'images_downloader/bugfixes' into maint