[d8] Improve error message for illegal --trace-path

Unconditionally check for errors (instead of a DCHECK), and print a
better error message (including the file path) if the file cannot be
opened.

R=tebbi@chromium.org
CC=machenbach@chromium.org

Bug: chromium:1087039
Change-Id: Ia5cb76b309e78631ecf9462de6c7cd3eb4bf9e59
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2224226Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68097}

src/d8/d8.cc

@@ -3601,8 +3601,14 @@ int Shell::Main(int argc, char* argv[]) {
   std::ofstream trace_file;
   if (options.trace_enabled && !i::FLAG_verify_predictable) {
     tracing = std::make_unique<platform::tracing::TracingController>();
-    trace_file.open(options.trace_path ? options.trace_path : "v8_trace.json");
-    DCHECK(trace_file.good());
+    const char* trace_path =
+        options.trace_path ? options.trace_path : "v8_trace.json";
+    trace_file.open(trace_path);
+    if (!trace_file.good()) {
+      printf("Cannot open trace file '%s' for writing: %s.\n", trace_path,
+             strerror(errno));
+      return 1;
+    }
 
 #ifdef V8_USE_PERFETTO
     // Set up the in-process backend that the tracing controller will connect