Refactor of error codes in import/export filters

I've changed KisImageBuilder_RESULT and ConversionStatus to KisImportExportErrorCode to unify error handling in import/export filters.

In the process I fixed some bugs and silent failures like:

• parsing errors in xcf library causing crash;
• saving tiff file to readonly location causing crash;
• saving exr file to readonly location causing std::exception and infinite waiting or silent failure;
• silent failure of saving spriter file to readonly location;
• crash when opening a preasumbly heightmap file that had 0 bytes;

I added tests that checks if import/export filters handles edge cases and broken files properly:

• saving to readonly location (no permission to write)
• opening from writeonly file (no permission to read)
• opening an empty file (incorrect format)

Test Plan

• Go to ./build/plugins/impex and run 'make test'. There are three tests that fails now:

  • ora (it has nearly no error handling, so it fails the test)
  • svg (the same as ora)
  • exr (make sure to click "OK", then the relevant test will pass - but there is still a test that fails on master and on my branch too which is not relevant to changes I've made, so 'make test' will show kis_exr_test as failed. Make sure to run 'exr/tests/kis_exr_test' to check if new tests pass)

• Open Krita and open/save files in different formats

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.