Texture names with spaces do not work

[ephb]

Hi,
If any material contains a texture which has a space in its name the conversion will fail without an error message.
This is basically two bugs.

Bug 1: Texture names are no correctly escaped so spaces in the name leads to the texture not being found.
Bug 2: Missing textures make the conversion fail without a correct error message.

Workaround: Use texture names without spaces
Please let me know if I can do anything to help resolve this.