[SPARK-2242] HOTFIX: pyspark shell hangs on simple job

This reverts a change introduced in 38702487, which redirected all stderr to the OS pipe instead of directly to the `bin/pyspark` shell output. This causes a simple job to hang in two ways:

1. If the cluster is not configured correctly or does not have enough resources, the job hangs without producing any output, because the relevant warning messages are masked.
2. If the stderr volume is large, this could lead to a deadlock if we redirect everything to the OS pipe. From the [python docs](https://docs.python.org/2/library/subprocess.html):

```
Note Do not use stdout=PIPE or stderr=PIPE with this function as that can deadlock
based on the child process output volume. Use Popen with the communicate() method
when you need pipes.
```

Note that we cannot remove `stdout=PIPE` in a similar way, because we currently use it to communicate the py4j port. However, it should be fine (as it has been for a long time) because we do not produce a ton of traffic through `stdout`.

That commit was not merged in branch-1.0, so this fix is for master only.

Author: Andrew Or <andrewor14@gmail.com>

Closes #1178 from andrewor14/fix-python and squashes the following commits:

e68e870 [Andrew Or] Merge branch 'master' of github.com:apache/spark into fix-python
20849a8 [Andrew Or] Tone down stdout interference message
a09805b [Andrew Or] Return more than 1 line of error message to user
6dfbd1e [Andrew Or] Don't swallow original exception
0d1861f [Andrew Or] Provide more helpful output if stdout is garbled
21c9d7c [Andrew Or] Do not mask stderr from output

python/pyspark/java_gateway.py

@@ -43,18 +43,23 @@ def launch_gateway():
             # Don't send ctrl-c / SIGINT to the Java gateway:
             def preexec_func():
                 signal.signal(signal.SIGINT, signal.SIG_IGN)
-            proc = Popen(command, stdout=PIPE, stdin=PIPE, stderr=PIPE, preexec_fn=preexec_func)
+            proc = Popen(command, stdout=PIPE, stdin=PIPE, preexec_fn=preexec_func)
         else:
             # preexec_fn not supported on Windows
-            proc = Popen(command, stdout=PIPE, stdin=PIPE, stderr=PIPE)
-        
+            proc = Popen(command, stdout=PIPE, stdin=PIPE)
+
         try:
             # Determine which ephemeral port the server started on:
-            gateway_port = int(proc.stdout.readline())
-        except:
-            error_code = proc.poll()
-            raise Exception("Launching GatewayServer failed with exit code %d: %s" %
-                (error_code, "".join(proc.stderr.readlines())))
+            gateway_port = proc.stdout.readline()
+            gateway_port = int(gateway_port)
+        except ValueError:
+            (stdout, _) = proc.communicate()
+            exit_code = proc.poll()
+            error_msg = "Launching GatewayServer failed"
+            error_msg += " with exit code %d!" % exit_code if exit_code else "! "
+            error_msg += "(Warning: unexpected output detected.)\n\n"
+            error_msg += gateway_port + stdout
+            raise Exception(error_msg)
 
         # Create a thread to echo output from the GatewayServer, which is required
         # for Java log output to show up: