ENH: Allow users to acknowledge all DICOM loading errors with a single click

Problem: Each DICOM loading error is displayed in a separate popup, which requires the user to click as many times as many series failed to load. In some cases there are a large number of failures (for example, experimental MRI sequences), which makes loading of a complete study very frustrating (need to click 30 times, waiting several seconds between each).

Solution: Changed the error reporting to collect all errors and show it to the user at the end of the loading in a single error popup.