Blender not being found can be reported via various errors (this should be
reworked in the 'blender finder API' at some point). `exec.ErrNotFound` is
returned when Blender cannot be found on `$PATH`, which is something that's
absolutely fine. This is now logged less dramatically.
Avoid the word "error" in logging when Blender cannot be found. Typically
these are warnings, and having the word "error" there makes people think
otherwise.
