A parameter is added to Tumbler's internal error signals to account for
the error domain, and the error code `TUMBLER_ERROR_OTHER_ERROR_DOMAIN`
is added to indicate that the error originated from a third-party API.

When sending the error signal via D-Bus, cancelling errors are filtered
out, and when the error originates from a third-party API, the error
message is prefixed with the domain and code of that error, in a
possibly parsable format.

Fixes #49.