Show error message if debug report generation errors.

If the process of generating a debug report in Kimchi fails while it
was executing (e.g. due to running out of disk space to generate the
report), the error message was not being shown in the Kimchi UI. Basically
src/kimchi/control/tasks.py was putting the error message in a JSON field
called message, while the UI was looking for the error message in a field
called reason.
Merge branches 'typo' and 'optimize' into devel