Blender Git Loki

Fix for lack of basic error checking in writing compressed .blend files
(which is enabled by default). If there was a problem reading or writing
in the compression process, the original .blend file could get lost. Now
errors are checked, and writing is done as follows:

write .blend@ -> compress .blend@ to [email protected] ->
rename [email protected] to .blend -> remove .blend@

We've had blender crash here, lose the original .blend and leave an
empty .blend@. It is not clear to me where this would happen in practice
if there is enough disk space and permissions are correct, so the actual
crash is likely not fixed by this commit.