Wang Qing a879302355 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

2018-09-29 22:10:57 +08:00

..

common

[java] put function meta in task spec and load functions with function meta (#2881)

2018-09-25 23:05:05 -07:00

global_scheduler

update ray cmake build process (#2853)

2018-09-12 11:19:33 -07:00

local_scheduler

[cmake] enable using thirdparty env variable to find installed dependency (#2912)

2018-09-23 07:52:33 -07:00

plasma

update ray cmake build process (#2853)

2018-09-12 11:19:33 -07:00

ray

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

2018-09-29 22:10:57 +08:00