[import] better error handling during the import

Description

Handle out-of-memory situations during the data import - in case there is not enough memory available on the system, show an appropriate message in the import dialog instead of crashing.

To reproduce such a crash, try to import a bigger file with a reduced amount of memory. For example, the import of the BLF files is very expensive at the moment (will be addressed in another MR) and can be used to easily reproduce the issues. For this, just import Matlab's demo file for BLF with after having started labplot with

prlimit --as=2000000000 ./labplot2

This MR also propagates other errors happening during the import, not necessarily related to the out of memory situations, to the ImportDialog so the appropriate messages can be shown to the user directly in the dialog instead of just writing out some debug output in the console.

Conformity

• [x ] Changelog entry
• Unit tests
• Update INSTALL
• Downport
• Fixes:

When external dependencies are removed

• Reporting to ...