[js-fuzzer] Slightly improve error messages

It took me a while to figure out how to set up everything to run the
fuzzer locally. These improved error messages would have helped to find
errors earlier.

R=machenbach@chromium.org

Bug: chromium:1142437
Change-Id: I414de0a8448cd81fdbf999cc7c3c8cf9394354f6
No-Try: true
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2543397
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71239}

tools/clusterfuzz/js_fuzzer/tools/run_one.py

@@ -22,6 +22,8 @@ BASE_PATH = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
 FOOZZIE = os.path.join(BASE_PATH, 'workdir', 'app_dir', 'v8_foozzie.py')
 TEST_CASES = os.path.join(BASE_PATH, 'workdir', 'output')
 
+assert os.path.exists(FOOZZIE)
+
 # Output pattern from foozzie.py when it finds a failure.
 FAILURE_RE = re.compile(
     r'# V8 correctness failure.'

tools/clusterfuzz/js_fuzzer/tools/workbench.py

@@ -37,8 +37,11 @@ RUN_ONE = os.path.join(BASE_PATH, 'tools', 'run_one.py')
 
 os.chdir(BASE_PATH)
 
-assert not os.path.exists(TEST_CASES)
-os.makedirs(TEST_CASES)
+if os.path.exists(TEST_CASES):
+  if not os.path.isdir(TEST_CASES) or os.listdir(TEST_CASES):
+    sys.exit("'output' must be an empty directory")
+else:
+  os.mkdir(TEST_CASES)
 
 # Use ~40000 for 24 hours of fuzzing on a modern work station.
 RUNS = 8