Be more robust

[bugaevc]

We should be able to recover from most of errors.

In particular, there's a lot of things that could go wrong when we try to copy our stdin to a temp file, such as issues with creating files in /tmp (perhaps fs permissions are misconfigured? perhaps /tmp is on a root filesystem that's mounted read-only?) and spawning programs (what if there's not enough memory to spawn cat?).

These errors should not be fatal to wl-copy/wl-paste.