Undefined name: from pyspark.util import _exception_message

[flake8](http://flake8.pycqa.org) testing of https://github.com/apache/spark on Python 3.7.0

$ __flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics__
```
./python/pyspark/java_gateway.py:172:20: F821 undefined name '_exception_message'
            emsg = _exception_message(e)
                   ^
1     F821 undefined name '_exception_message'
1
```
@HyukjinKwon

Author: cclauss

python/pyspark/java_gateway.py

@@ -34,6 +34,7 @@
 from py4j.java_gateway import java_import, JavaGateway, JavaObject, GatewayParameters
 from pyspark.find_spark_home import _find_spark_home
 from pyspark.serializers import read_int, write_with_length, UTF8Deserializer
+from pyspark.util import _exception_message
 
 
 def launch_gateway(conf=None):