changed the texture-not-found exception to be more terse

the explanation is nice, but really ought to be done in the program
*using* the texture loader, not the texture loader itself. For
development, the extra message just makes it harder to find the path
that caused the error.
obj loader fixes, and more JSON block conversions
added optional world reloader to wxexplorer
stub biome coloring support (no lookups yet)
removed (now empty) python block definition subpackage