Only delete problem zip after detecting problem directory

Otherwise, if you only have the zip, it gets deleted and then the
script fails, leaving you without the data of that problem.
In my case, I didn't notice, and ran `import-contest` again after
unzipping all problem archives, which then failed because the
first problem was missing.

Author: eldering

misc-tools/import-contest.in

@@ -216,11 +216,11 @@ if os.path.exists('problems.yaml') or os.path.exists('problems.json') or os.path
         confirmIndividually = dj_utils.confirm("Confirm individually for every problem", False)
         for problem in problems:
             print(f'\nPreparing problem \'{problem}\'.')
-            if os.path.exists(f'{problem}.zip'):
-                os.unlink(f'{problem}.zip')
             if not os.path.isdir(problem) or not os.path.isfile(f'{problem}/problem.yaml'):
                 print('Problem directory not found or doesn\'t contain a problem.yaml.')
                 exit(3)
+            if os.path.exists(f'{problem}.zip'):
+                os.unlink(f'{problem}.zip')
             zip_command = f"zip -r '../{problem}' -- .timelimit *"
             process = subprocess.Popen(zip_command, cwd=problem, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, text=True, shell=True)