Improved exception handling when importing the module

[piglei]

The current expection handling is too vague, and in certain circumstances, the error message may confuse the user.

For example, if an error occurs while importing the "_mysql" module, the original error message is as follows:

  File "MySQLdb/__init__.py", line 18, in <module>
    from . import _mysql
ImportError: /lib64/libstdc++.so.6: cannot allocate memory in static TLS block

But on the user side, he can only see the exception message like this:

/MySQLdb/__init__.py", line 24, in <module>
    version_info, _mysql.version_info, _mysql.__file__
NameError: name '_mysql' is not defined

This PR fixes this issue by making the exception handling statements more precise.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.13 🎉

Comparison is base (1f4fb4d) 86.15% compared to head (5996fe2) 86.28%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #596      +/-   ##
==========================================
+ Coverage   86.15%   86.28%   +0.13%     
==========================================
  Files           6        6              
  Lines         549      547       -2     
==========================================
- Hits          473      472       -1     
+ Misses         76       75       -1     

Impacted Files	Coverage Δ
MySQLdb/__init__.py	86.84% <100.00%> (+1.84%)	⬆️

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