Toughen {{canonexp}} camera controller and pass errors back to caller.

While switching back and forth between users and programs during initial engineering, we left a canonexp /tmp file owned by a non-fps user and non-group writable. This caused canonexp to segfault.

This can easily be hardened:

open("/tmp/cameradump.raw", O_WRONLY|O_CREAT|O_TRUNC, 0666) = -1 EACCES (Permission denied)
--- SIGSEGV {si_signo=SIGSEGV, si_code=SEGV_MAPERR, si_addr=0} ---