machines: Fix D-Bus error messages
machines: Fix D-Bus error messages

`ex` is an object and thus not directly printable. Consistently turn it
into a string so that the warning becomes useful. Other places already
used `JSON.stringify()`, which works, but is a bit harder to read.

Also use catch() consistently.

Closes #13943
cockpit-project/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,cockpituous/cockpit,mvollmer/cockpit,cockpituous/cockpit,deryni/cockpit,garrett/cockpit,cockpit-project/cockpit,martinpitt/cockpit,deryni/cockpit,garrett/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpituous/cockpit,deryni/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpituous/cockpit,deryni/cockpit,deryni/cockpit,moraleslazaro/cockpit,garrett/cockpit,mvollmer/cockpit,mvollmer/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,deryni/cockpit,garrett/cockpit,cockpit-project/cockpit,martinpitt/cockpit,cockpituous/cockpit,moraleslazaro/cockpit,deryni/cockpit,garrett/cockpit,moraleslazaro/cockpit,cockpituous/cockpit,moraleslazaro/cockpit