Improve log message when failing to fork worker process (#2990)

## What do these changes do?
```c++
  // Try to execute the worker command.
  int rv = execvp(worker_command_args[0],
                  const_cast<char *const *>(worker_command_args.data()));
  // The worker failed to start. This is a fatal error.
  RAY_LOG(FATAL) << "Failed to start worker with return value " << rv;
```
When starting a process fails, the return value `rv` always be set to -1.
It is useless for us.
The log message should show some meaningful infos.

For example, If we did't install java. The message showed for us should be:
```shell
 Failed to start worker: No such file or directory.
```
This could help us to locate issue quickly.

## Related issue number
N/A

src/ray/raylet/worker_pool.cc

@@ -118,12 +118,18 @@ void WorkerPool::StartWorkerProcess(const Language &language) {
 
   // Launch the process to create the worker.
   pid_t pid = fork();
-  if (pid != 0) {
+  if (pid < 0) {
+    // Failure case.
+    RAY_LOG(FATAL) << "Failed to fork worker process: " << strerror(errno);
+    return;
+  } else if (pid > 0) {
+    // Parent process case.
     RAY_LOG(DEBUG) << "Started worker process with pid " << pid;
     starting_worker_processes_.emplace(std::make_pair(pid, num_workers_per_process_));
     return;
   }
 
+  // Child process case.
   // Reset the SIGCHLD handler for the worker.
   signal(SIGCHLD, SIG_DFL);
 
@@ -138,7 +144,8 @@ void WorkerPool::StartWorkerProcess(const Language &language) {
   int rv = execvp(worker_command_args[0],
                   const_cast<char *const *>(worker_command_args.data()));
   // The worker failed to start. This is a fatal error.
-  RAY_LOG(FATAL) << "Failed to start worker with return value " << rv;
+  RAY_LOG(FATAL) << "Failed to start worker with return value " << rv << ": "
+                 << strerror(errno);
 }
 
 void WorkerPool::RegisterWorker(std::shared_ptr<Worker> worker) {