redshift-gtk: Show errors from child process in a dialog

This dialog is shown whenever the child process exits with a
non-zero status. The error output from stderr of the child
process is buffered in redshift-gtk in case the child exits
unexpectedly.
redshift-gtk: Remove unnecessary semi-colons at end-of-line
redshift-gtk: Ignore exception in termwait when child is gone

The termwait method is used to wait until the child process has
quit. Previously this would fail with an exception if the child
has already quit. This is now ignored.