Handle missing materials in obj2pbrt.
Handle missing materials in obj2pbrt.

Add a semi-helpful comment in the generated pbrt file (instead of crashing)
if a material definition isn't available.

Fix via stephenlombardi@, issue #80.

nyue/pbrt-v3,shihchinw/pbrt-v3,mmp/pbrt-v3,mmp/pbrt-v3,mmp/pbrt-v3,nispaur/pbrt-v3,nyue/pbrt-v3,mmp/pbrt-v3,shihchinw/pbrt-v3,nispaur/pbrt-v3,nispaur/pbrt-v3,nyue/pbrt-v3,ambakshi/pbrt-v3,shihchinw/pbrt-v3,ambakshi/pbrt-v3,ambakshi/pbrt-v3,nispaur/pbrt-v3,nyue/pbrt-v3,shihchinw/pbrt-v3