[SPARK-22043][PYTHON] Improves error message for show_profiles and dump_profiles

[HyukjinKwon]

What changes were proposed in this pull request?

This PR proposes to improve error message from:

>>> sc.show_profiles()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../spark/python/pyspark/context.py", line 1000, in show_profiles
    self.profiler_collector.show_profiles()
AttributeError: 'NoneType' object has no attribute 'show_profiles'
>>> sc.dump_profiles("/tmp/abc")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../spark/python/pyspark/context.py", line 1005, in dump_profiles
    self.profiler_collector.dump_profiles(path)
AttributeError: 'NoneType' object has no attribute 'dump_profiles'

to

>>> sc.show_profiles()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../spark/python/pyspark/context.py", line 1003, in show_profiles
    raise RuntimeError("'spark.python.profile' configuration must be set "
RuntimeError: 'spark.python.profile' configuration must be set to 'true' to enable Python profile.
>>> sc.dump_profiles("/tmp/abc")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File ".../spark/python/pyspark/context.py", line 1012, in dump_profiles
    raise RuntimeError("'spark.python.profile' configuration must be set "
RuntimeError: 'spark.python.profile' configuration must be set to 'true' to enable Python profile.

How was this patch tested?

Unit tests added in python/pyspark/tests.py and manual tests.

[SparkQA]

Test build #81855 has finished for PR 19260 at commit 55c7094.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[HyukjinKwon]

cc @maver1ck and @viirya who I believe recently ran Python profile. Could you take a look please when you have some time?

[maver1ck]

LGTM

[viirya]

LGTM

[HyukjinKwon]

Thanks @maver1ck, @felixcheung and @viirya.

[HyukjinKwon]

Merged to master, branch-2.2 and branch-2.1.