fix(C++): centralize ReadFileToTable error handling

[SYaoJun]

Reason for this PR

Currently, the ReadFileToTable has duplicated error handling in two conditions.
A better way is centralizing the check statements out of the if condition.

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

❌ Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.22%. Comparing base (eab0fa5) to head (695abc2).
⚠️ Report is 28 commits behind head on main.

Files with missing lines	Patch %	Lines
cpp/src/graphar/filesystem.cc	66.66%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #866      +/-   ##
============================================
+ Coverage     80.21%   80.22%   +0.01%     
  Complexity      615      615              
============================================
  Files            93       93              
  Lines         10258    10254       -4     
  Branches       1050     1049       -1     
============================================
- Hits           8228     8226       -2     
+ Misses         1790     1788       -2     
  Partials        240      240              

Flag	Coverage Δ
cpp	71.52% <66.66%> (+0.01%)	⬆️

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[Copilot]

Pull request overview

This PR improves code quality through refactoring and spelling corrections. The main change centralizes duplicated error handling in the ReadFileToTable function, while additional changes fix typos in example code comments.

Changes:

• Refactored ReadFileToTable to eliminate duplicated error handling by declaring the status variable once and checking it after the conditional branches
• Fixed spelling of "specified" (was "specificed") in mid-level reader example output messages
• Fixed spelling of "construct" (was "constuct") and "first" (was "fisrt") in low-level reader example comments

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
cpp/src/graphar/filesystem.cc	Centralized error handling by extracting duplicate status checks from if-else branches
cpp/examples/mid_level_reader_example.cc	Fixed spelling of "specified" in console output messages and removed extra line break
cpp/examples/low_level_reader_example.cc	Fixed spelling errors in comments ("construct" and "first")

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