fix: optimizing ClassNotFoundException error message for MYSQL

[Z-HUANT]

Purpose of the PR

According to the documentation at https://hugegraph.apache.org/cn/docs/contribution-guidelines/hugegraph-server-idea-setup/, when setting up the development environment for the Server and using MySQL as the database, if the MySQL driver package is missing, the startup will fail. However, the error message displayed is not clear. This submission optimizes the error message.

Main Changes

Modified ClassNotFoundException exception error message.

Verifying these changes

• Trivial rework / code cleanup without any test coverage. (No Need)
• Already covered by existing tests, such as (please modify tests here).
• Need tests and can be verified as follows:

First, modify the "hugegraph.properties" configuration file to use MySQL as the database. Start the program without importing the MySQL driver package and check the error message.

Does this PR potentially affect the following parts?

• Nope
• Dependencies (add/update license info)
• Modify configurations
• The public API
• Other affects (typed here)

Updated error message.

Documentation Status

• Doc - TODO
• Doc - Done
• Doc - No Need

[codecov]

Codecov Report

Merging #2246 (7ec7306) into master (8ee3d32) will increase coverage by 3.55%.
The diff coverage is 0.00%.

@@             Coverage Diff              @@
##             master    #2246      +/-   ##
============================================
+ Coverage     65.03%   68.59%   +3.55%     
+ Complexity      979      977       -2     
============================================
  Files           498      498              
  Lines         40682    40682              
  Branches       5681     5681              
============================================
+ Hits          26458    27904    +1446     
+ Misses        11596    10075    -1521     
- Partials       2628     2703      +75     

Impacted Files	Coverage Δ
...n/java/org/apache/hugegraph/StandardHugeGraph.java	69.15% <0.00%> (+1.32%)	⬆️
...e/hugegraph/backend/store/mysql/MysqlSessions.java	69.49% <0.00%> (ø)

... and 67 files with indirect coverage changes

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[imbajin]

lgtm