Fix job operation crash on error

Don't clear error after g_propagate_error, because it's already invalid.

Steps to reproduce:

1. Open a folder with a file
2. Rename file in Thunar
3. Rename renamed file in terminal
4. Undo rename in Thunar
5. Message with corrupted text is displayed, then crash.

mentioned in commit e8da3e3d

merged

Indeed, that error clearing was too much, thank you for the fix !

mentioned in commit cf32be20

cherry-picked to 4.18 branch

changed milestone to %4.18.7