### What changes were proposed in this pull request?
The str of `CapaturedException` is now returned by str(self.desc) rather than repr(self.desc), which is more user-friendly. It also handles unicode under python2 specially.
### Why are the changes needed?
This is an improvement, and makes exception more human readable in python side.
### Does this PR introduce any user-facing change?
Before this pr, select `中文字段` throws exception something likes below:
```
Traceback (most recent call last):
File "/Users/advancedxy/code_workspace/github/spark/python/pyspark/sql/tests/test_utils.py", line 34, in test_capture_user_friendly_exception
raise e
AnalysisException: u"cannot resolve '`\u4e2d\u6587\u5b57\u6bb5`' given input columns: []; line 1 pos 7;\n'Project ['\u4e2d\u6587\u5b57\u6bb5]\n+- OneRowRelation\n"
```
after this pr:
```
Traceback (most recent call last):
File "/Users/advancedxy/code_workspace/github/spark/python/pyspark/sql/tests/test_utils.py", line 34, in test_capture_user_friendly_exception
raise e
AnalysisException: cannot resolve '`中文字段`' given input columns: []; line 1 pos 7;
'Project ['中文字段]
+- OneRowRelation
```
### How was this patch
Add a new test to verify unicode are correctly converted and manual checks for thrown exceptions.
This pr's credits should go to uncleGen and is based on https://github.com/apache/spark/pull/17267Closes#25814 from advancedxy/python_exception_19926_and_21045.
Authored-by: Xianjin YE <advancedxy@gmail.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
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