Exit gen_snapshot and log an error if an output file path is invalid

runtime/bin/gen_snapshot.cc

Index: runtime/bin/gen_snapshot.cc
diff --git a/runtime/bin/gen_snapshot.cc b/runtime/bin/gen_snapshot.cc
index cf1ba385e2362470dfa58e6859159d3b17b22836..05452ae26dffa59dbc800927030c40596ac31866 100644
--- a/runtime/bin/gen_snapshot.cc
+++ b/runtime/bin/gen_snapshot.cc
@@ -382,7 +382,12 @@ static void WriteSnapshotFile(const char* filename,
                               const uint8_t* buffer,
                               const intptr_t size) {
   File* file = File::Open(filename, File::kWriteTruncate);
-  ASSERT(file != NULL);
+  if (file == NULL) {
+    Log::PrintErr("Error: Unable to write snapshot file: %s\n\n", filename);
+    Dart_ExitScope();
+    Dart_ShutdownIsolate();
+    exit(kErrorExitCode);
+  }
   if (!file->WriteFully(buffer, size)) {
     Log::PrintErr("Error: Failed to write snapshot file.\n\n");
   }