Errors due to invalid Json files are not properly handled in the UI

In case there is an error in the Json file defining the kits, this is not being properly explained to the use. Such error can happen if someone accidentally uses the QtForMCUs 2.2 files as a 2.3 version.

Screenshot2 shows that there is no indication what the problem is.

Screenshot3 shows a small improvement that at least shows some errors in the messages.

Screenshot4 is the intended solution, but was not completed due to lack of time. The code is attached in the patch file.

The general problem here is that the MCU plugin is not set up to handle errors well.