Reinoldrojas/bundle location save error message

Display an error message when trying to create a bundle with an empty save location.

Found from the Krita-artists beta testers here

Test Plan

go to settings -> manage resources -> create a bundle -> Click "save to" button -> cancel -> save ( Error Dialog should popup here)

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.