Debug errors were not shown anywhere in the GUI, they were just
ignored. This commit adds new signal for those debug errors and
directs them to checking log.
Solves ticket #1898 (GUI: Handle internal errors from lib)
This commit modifies and registers ErrorItem as proper metatype so it
can be used with Qt signals. Then sending Error-signals is refactored
so that ErrorItem instances are sent instead of several different
arguments that already contained couple of lists.