Update ldraw_import.py to find the parts directories on Case-Sensitive File Systems (ie. Linux, OSX)

Patch to allow finding of the ldraw parts library on case-sensitive file systems, (hopefully) without breaking compatibility on case-insensitive file systems.

Previously, this would fail if any directories in the ldraw path had capital letters in them.  I made this patch to get ldraw_import.py to work on my computer.

Also, there is a lot of error handling that needs to be fixed, they try to use variables that do not exist yet. I'll work on that next.