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

## 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.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #19260 from HyukjinKwon/profile-errors.

(cherry picked from commit 7c726620)
Signed-off-by: hyukjinkwon <gurwls223@gmail.com>